Replace interp_set_temp with scoped_restore_interp

Replace interp_set_temp with scoped_restore_interp

This removes interp_set_temp and an associated cleanup, in favor of a
new RAII class, scoped_restore_interp.

ChangeLog
2017-09-11  Tom Tromey  <tom@tromey.com>

	* cli/cli-script.c (restore_interp): Remove.
	(read_command_lines): Use scoped_restore_interp.
	* interps.c (scoped_restore_interp::set_temp): Rename from
	interp_set_temp.
	* interps.h (class scoped_restore_interp): New.
	(interp_set_temp): Remove.