Make native gdbserver boards no longer be "remote" (in DejaGnu terms)

Make native gdbserver boards no longer be "remote" (in DejaGnu terms)

This commit finally clears the "isremote" flag in the native-gdbserver
and native-stdio-gdbserver boards.  The goal is to make all "native"
boards be considered not remote in DejaGnu terms, like the
native-extended-gdbserver board is too.

DejaGnu automatically considers boards remote if their names don't
match the local hostname.  That means that native-gdbserver and
native-extended-gdbserver are considered remote by default by DejaGnu,
even though they run locally.  native-extended-gdbserver, however,
overrides its isremote flag to force it to be not remote.  So we are
in that weird state where native-gdbserver is considered remote, and
native-extended-gdbserver is considered not remote.

A recent set of commits fixed all the problems (and some more) exposed
by testing with --target_board=native-gdbserver and
--target_board=native-stdio-gdbserver with isremote forced off on
x86-64 GNU/Linux.  I believe we're good to go now.

The native-stdio-gdbserver.exp/remote-stdio-gdbserver.exp boards
required deep non-obvious modifications unfortunately...  The problem
is that if a board is not remote, then DejaGnu doesn't call
${board}_spawn / ${board}_exec at all, and the
native-stdio-gdbserver.exp board relies on those procedures being
called.  To fix that, this commit redesigns how the stdio boards hook
into the testing framework to spawn gdbserver.  IMO, this is a good
change anyway, because the way its done currently is a bit of a hack,
and the result turns out to be simpler, even.  With this commit, they
now no longer load the "gdbserver" generic config, and hook at the
mi_gdb_target_load/gdb_reload level instead, making them more like
traditional board files.

To share code between native-stdio-gdbserver.exp and
remote-stdio-gdbserver.exp, a new shared stdio-gdbserver-base.exp file
is created.

Instead of having each native board clear isremote manually, boards
source the new "local-board.exp" file.

This also adds a new section to testsuite/README file discussing
local/remote/native, so that we can easily refer to it.

gdb/testsuite/ChangeLog:
2017-10-16  Pedro Alves  <palves@redhat.com>
	    Simon Marchi  <simon.marchi@polymtl.ca>

	* README (Local vs Remote vs Native): New section.
	* boards/local-board.exp: New file, with bits factored out from
	...
	* boards/native-extended-gdbserver.exp: ... here.  Load
	"local-board".
	* boards/native-gdbserver.exp: Load "local-board".
	(${board}_spawn, ${board}_exec): Delete.
	* boards/native-stdio-gdbserver.exp: Most contents factored out to
	...
	* boards/stdio-gdbserver-base.exp: ... this new file.
	* boards/native-stdio-gdbserver.exp: Reimplement, by loading
	"stdio-gdbserver-base" and defining a get_target_remote_pipe_cmd
	procedure.
	* boards/remote-stdio-gdbserver.exp: Load stdio-gdbserver-base
	instead of native-stdio-gdbserver.  Don't set gdb_server_prog nor
	stdio_gdbserver_command.
	(${board}_get_remote_address, ${board}_get_comm_port)
	(${board}_download, ${board}_upload): Delete.
	(get_target_remote_pipe_cmd): New.