Use scoped_fd in more places

This changes a few more places to use scoped_fd.  This allows the
removal of some cleanups.

Regression tested by the buildbot, though note that I'm not sure
whether the buildbot actually builds anything using all of these
files.

gdb/ChangeLog
2018-03-08  Tom Tromey  <tom@tromey.com>

	* source.c (get_filename_and_charpos): Use scoped_fd.
	* nto-procfs.c (procfs_open_1): Use scoped_fd.
	(procfs_pidlist): Likewise.
	* procfs.c (proc_get_LDT_entry): Use scoped_fd.
	(iterate_over_mappings): Likewise.