ui/ncurses: Exit safe mode on configuration change

Selecting 'OK' in the configuration or language select screen will send
a configuration change to the server and exit safe mode.
Exit safe mode explicity and warn the user that they are about to exit
it.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>