comm: Make udp_make_socket be able to tolerate failures

Previously, it would log errors with fatal or fatal_perror.  Now it
takes a message class and uses lg_perror, and also returns a boolean
to let the caller know whether it worked.

The repetitive calls to fatal_perror in udp_make_socket have been
replaced with a couple of macros.

The one existing call site passes M_FATAL.  So no substantial
functional change in this patch.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>