Implement the ability to set/unset environment variables to GDBserver when starting...

Implement the ability to set/unset environment variables to GDBserver when starting the inferior

This patch implements the ability to set/unset environment variables
on the remote target, mimicking what GDB already offers to the user.
There are two features present here: user-set and user-unset
environment variables.

User-set environment variables are only the variables that are
explicitly set by the user, using the 'set environment' command.  This
means that variables that were already present in the environment when
starting GDB/GDBserver are not transmitted/considered by this feature.

User-unset environment variables are variables that are explicitly
unset by the user, using the 'unset environment' command.

The idea behind this patch is to store user-set and user-unset
environment variables in two separate sets, both part of gdb_environ.
Then, when extended_remote_create_inferior is preparing to start the
inferior, it will iterate over the two sets and set/unset variables
accordingly.  Three new packets are introduced:

- QEnvironmentHexEncoded, which is used to set environment variables,
  and contains an hex-encoded string in the format "VAR=VALUE" (VALUE
  can be empty if the user set a variable with a null value, by doing
  'set environment VAR=').

- QEnvironmentUnset, which is used to unset environment variables, and
  contains an hex-encoded string in the format "VAR".

- QEnvironmentReset, which is always the first packet to be
  transmitted, and is used to reset the environment, i.e., discard any
  changes made by the user on previous runs.

The QEnvironmentHexEncoded packet is inspired on LLDB's extensions to
the RSP.  Details about it can be seen here:

  <https://raw.githubusercontent.com/llvm-mirror/lldb/master/docs/lldb-gdb-remote.txt>

I decided not to implement the QEnvironment packet because it is
considered deprecated by LLDB.  This packet, on LLDB, serves the same
purpose of QEnvironmentHexEncoded, but sends the information using a
plain text, non-hex-encoded string.

The other two packets are new.

This patch also includes updates to the documentation, testsuite, and
unit tests, without introducing regressions.

gdb/ChangeLog:
2017-08-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* NEWS (Changes since GDB 8.0): Add entry mentioning new support
	for setting/unsetting environment variables on the remote target.
	(New remote packets): Add entries for QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset.
	* common/environ.c (gdb_environ::operator=): Extend method to
	handle m_user_set_env_list and m_user_unset_env_list.
	(gdb_environ::clear): Likewise.
	(match_var_in_string): Change type of first parameter from 'char
	*' to 'const char *'.
	(gdb_environ::set): Extend method to handle
	m_user_set_env_list and m_user_unset_env_list.
	(gdb_environ::unset): Likewise.
	(gdb_environ::clear_user_set_env): New method.
	(gdb_environ::user_set_envp): Likewise.
	(gdb_environ::user_unset_envp): Likewise.
	* common/environ.h (gdb_environ): Handle m_user_set_env_list and
	m_user_unset_env_list on move constructor/assignment.
	(unset): Add new default parameter 'update_unset_list = true'.
	(clear_user_set_env): New method.
	(user_set_envp): Likewise.
	(user_unset_envp): Likewise.
	(m_user_set_env_list): New std::set.
	(m_user_unset_env_list): Likewise.
	* common/rsp-low.c (hex2str): New function.
	(bin2hex): New overload for bin2hex function.
	* common/rsp-low.c (hex2str): New prototype.
	(str2hex): New overload prototype.
	* remote.c: Include "environ.h". Add QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset.
	(remote_protocol_features): Add QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset packets.
	(send_environment_packet): New function.
	(extended_remote_environment_support): Likewise.
	(extended_remote_create_inferior): Call
	extended_remote_environment_support.
	(_initialize_remote): Add QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset packet configs.
	* unittests/environ-selftests.c (gdb_selftest_env_var):
	New variable.
	(test_vector_initialization): New function.
	(test_init_from_host_environ): Likewise.
	(test_reinit_from_host_environ): Likewise.
	(test_set_A_unset_B_unset_A_cannot_find_A_can_find_B):
	Likewise.
	(test_unset_set_empty_vector): Likewise.
	(test_vector_clear): Likewise.
	(test_std_move): Likewise.
	(test_move_constructor):
	(test_self_move): Likewise.
	(test_set_unset_reset): Likewise.
	(run_tests): Rewrite in terms of the functions above.

gdb/gdbserver/ChangeLog:
2017-08-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* server.c (handle_general_set): Handle QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset packets.
	(handle_query): Inform remote that QEnvironmentHexEncoded,
	QEnvironmentUnset and QEnvironmentReset are supported.

gdb/doc/ChangeLog:
2017-08-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (set environment): Add @anchor.  Explain that
	environment variables set by the user are sent to GDBserver.
	(unset environment): Likewise, but for unsetting variables.
	(Connecting) <Remote Packet>: Add "environment-hex-encoded",
	"QEnvironmentHexEncoded", "environment-unset", "QEnvironmentUnset",
	"environment-reset" and "QEnvironmentReset" to the table.
	(Remote Protocol) <QEnvironmentHexEncoded, QEnvironmentUnset,
	QEnvironmentReset>: New item, explaining the packet.

gdb/testsuite/ChangeLog:
2017-08-31  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.base/share-env-with-gdbserver.c: New file.
	* gdb.base/share-env-with-gdbserver.exp: Likewise.