Change open_terminal_stream to return a gdb_file_up

Change open_terminal_stream to return a gdb_file_up

This changes open_terminal_stream to return a gdb_file_up, eliminating
another use of make_cleanup_fclose.  Arguably perhaps new_ui should
take ownership of the files using a move, but there is at least one
spot where this isn't appropriate (or at least not currently done), so
I elected to use a more minimal approach.

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

	* top.c (open_terminal_stream): Return gdb_file_up.
	(new_ui_command): Update.