Use target_terminal_ours_for_output in cp-support.c

Use target_terminal_ours_for_output in cp-support.c

We're only doing output here, so leave raw/cooked mode alone, as well
as the SIGINT handler.

Restore terminal settings after output, while at it.

gdb/ChangeLog:
2016-04-12  Pedro Alves  <palves@redhat.com>

	* cp-support.c (gdb_demangle): Use target_terminal_ours_for_output
	instead of target_terminal_ours, and restore target terminal with
	a cleanup.