Replace do_restore_instream_cleanup with scoped_restore

Replace do_restore_instream_cleanup with scoped_restore

This changes the users of do_restore_instream_cleanup to use a
scoped_restore instead.  This patch is broken out because it warrants
some additional attention: in particular it's unclear to me whether
current_ui can change in the body of these functions -- but if it can,
then the cleanup would have modified a different UI's instream member.

ChangeLog
2017-08-03  Tom Tromey  <tom@tromey.com>

	* top.h (do_restore_instream_cleanup): Remove.
	* top.c (do_restore_instream_cleanup): Remove.
	(read_command_file): Use scoped_restore.
	* cli/cli-script.c (execute_user_command): Use scoped_restore.