Use std::string rather than dyn-string

Use std::string rather than dyn-string

This patch changes some code in cli-cmds.c to use std::string rather
than dyn-string, removing some cleanups.  Since this was the last use
of dyn-string in gdb, this patch also removes
make_cleanup_dyn_string_delete.

2016-09-23  Tom Tromey  <tom@tromey.com>

	* utils.h (make_cleanup_dyn_string_delete): Remove declaration.
	* utils.c: Don't include dyn-string.h.
	(do_dyn_string_delete, make_cleanup_dyn_string_delete): Remove.
	* cli/cli-cmds.c: Include <string>.  Don't include dyn-string.h.
	(argv_to_string): Rename.  Change return type to std::string.
	(alias_command): Use std::string.