Display the ">" prompt in interactive mode while reading canned

Display the ">" prompt in interactive mode while reading canned
commands, even when the current interpreter is MI.

 interps.c (interp_set_temp): New function.
 interps.h (interp_set_temp): Add prototype.
 cli/cli-script.c (restore_interp): New cleanup function.
 (read_command_lines): Temporarily override the current interpreter
 with CLI and arrange for restoring the original one.