Fix PR gdb/21954: make 'unset environment' work again

Fix PR gdb/21954: make 'unset environment' work again

When I made commit 9a6c7d9c021cfeb290d76584db7a01e57e7c3d4e, which
C++-fied gdb/common/environ.[ch], I mistakenly altered the behaviour
of the 'unset environment' command.  This command, which should delete
all environment variables, is now resetting the list of variables to
the state they were when GDB was started.

This commit fixes this regression, and also adds a test on
gdb.base/environ.exp which really checks if 'unset environment'
worked.

gdb/ChangeLog:
2017-08-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21954
	* infcmd.c (unset_environment_command): Use the 'clear' method on
	the environment instead of resetting it.

gdb/testsuite/ChangeLog:
2017-08-15  Sergio Durigan Junior  <sergiodj@redhat.com>

	PR gdb/21954
	* gdb.base/environ.exp: Add test to check if 'unset environment'
	works.