ui/ncurses/nc-widgets: Add initial textbox validation functions

We'd like to do some validation of the system configuration parameters,
so add a few validation types to the widget code. We currently need
integer, ipv4 address and multiple ipv4 address types. These are
implemented as small wrappers around the ncurses form validator code.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>