vi: allow manual screen update if SIGWINCH isn't supported

vi: allow manual screen update if SIGWINCH isn't supported

On platforms that don't support SIGWINCH vi can be configured
with FEATURE_VI_USE_SIGNALS disabled and FEATURE_VI_WIN_RESIZE
enabled.  This allows the user to force an update with ^L when
the screen is resized.

However, because the SIGWINCH handler hasn't run the virtual
screen buffer won't have been updated and the display becomes
corrupted.  Fix this by calling new_screen() if necessary.

Signed-off-by: Ron Yorston <rmy@pobox.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>