Remove do_closedir_cleanup

Remove do_closedir_cleanup

This removes both copies of do_closedir_cleanup in favor of a new
unique_ptr specialization.

Tested by the buildbot, though I'm not sure that these code paths are
exercised there.

ChangeLog
2018-05-04  Tom Tromey  <tom@tromey.com>

	* nto-procfs.c (do_closedir_cleanup): Remove.
	(procfs_pidlist): Use gdb_dir_up.
	* procfs.c (do_closedir_cleanup): Remove.
	(proc_update_threads): Use gdb_dir_up.
	* common/filestuff.h (struct gdb_dir_deleter): New.
	(gdb_dir_up): New typedef.