Make check_for_argument skip whitespace after arg itself

Make check_for_argument skip whitespace after arg itself

Basically every caller of check_for_argument needs to skip space after
the argument.  This patch makes check_for_argument do it itself.

Suggested by Philippe Waroquiers.

gdb/ChangeLog:
2019-06-13  Pedro Alves <palves@redhat.com>

	* ax-gdb.c (agent_command_1): Remove skip_spaces call.
	* breakpoint.c (watch_maybe_just_location): Remove skip_spaces
	call.
	* cli/cli-cmds.c (apropos_command): Remove skip_spaces call.
	* cli/cli-utils.c (extract_info_print_args): Remove skip_spaces
	calls.
	(check_for_argument): Skip spaces after argument.