Remove some cleanups from stack.c

Remove some cleanups from stack.c

This removes some cleanups from stack.c by using std::string or
gdb::unique_xmalloc_ptr.  One cleanup remains in this file; I did not
remove it here because it is handled in another patch series that has
yet to be resolved.

gdb/ChangeLog
2017-09-29  Tom Tromey  <tom@tromey.com>

	* stack.c (parse_frame_specification): Use std::string
	(info_frame_command): Use gdb::unique_xmalloc_ptr.