gdb: Add command completers for some info commands

gdb: Add command completers for some info commands

Add command completion for info variables, functions, args, and
locals.  This completer only completes the command line options as
these commands all take a regexp which GDB can't really offer
completions for.

gdb/ChangeLog:

	* cli/cli-utils.c (info_print_command_completer): New function.
	* cli/cli-utils.h: Add 'completer.h' include, and forward
	declaration for 'struct cmd_list_element'.
	(info_print_command_completer): Declare.
	* stack.c (_initialize_stack): Add completer for 'info locals' and
	'info args'.
	* symtab.c (_initialize_symtab): Add completer for 'info
	variables' and 'info functions'.
	* NEWS: Mention completion for additional info commands.