[binutils-gdb/gdb-7.12-branch] Allow resetting an empty inferior-tty

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 2b3eaab2c3eadd12bbd56499c00dca8cefb81552 ***

Author: Simon Marchi <simon.marchi@ericsson.com>
Branch: gdb-7.12-branch
Commit: 2b3eaab2c3eadd12bbd56499c00dca8cefb81552

Allow resetting an empty inferior-tty

This patch allows the user to set the inferior-tty to "empty", in order
to come back to the default behaviour of using the same tty as gdb is
using.

This is already supported in MI (and tested in gdb.mi/mi-basics.exp).

I added a new test, set-inferior-tty.exp, where I test only the setting
and unsetting of the parameter.  It would be nice to actually test that
the inferior output properly goes to the separate tty, but that will be
for another day.

gdb/ChangeLog:

	* infcmd.c (set_inferior_io_terminal): Set inferior terminal to
	NULL if terminal_name is an empty string.
	(_initialize_infcmd): Make the argument of "set inferior-tty"
	optional, mention it in the help doc.

gdb/doc/ChangeLog:

	* gdb.texinfo (Input/Output): Mention possibility to unset
	inferior-tty.

gdb/testsuite/ChangeLog:

	* gdb.base/set-inferior-tty.exp: New file.
	* gdb.base/set-inferior-tty.c: New file.