Make gdb.parameter("directories") work.

	Make gdb.parameter("directories") work.
	New command "set directories".
	* NEWS: Document them.
	* source.c (set_directories_command): New function.
	(show_directories_1): Renamed from show_directories.
	All callers updated.
	(show_directories_command): New function.
	(_initialize_source): Install "directories" as a set/show
	variable instead of just a show command.

	doc/
	* gdb.texinfo (Source Path): Document "set directories".

	testsuite/
	* gdb.base/help.exp: Update expected output.
	* gdb.python/py-parameter.exp: New file.