Generate syslog warnings for %s fmt strings NULL to "(null)" conversions.

Generate syslog warnings for %s fmt strings NULL to "(null)" conversions.
Over time we can repair software which performs this non-standard behaviour,
and fix bugs along the way.  Let's first find out how bad the situation is
by deploying this in snapshots.

This type of logging is possible because OpenBSD syslog_r(3) -> sendsyslog(2)
is side-effect free enough to be used in the bowels of libc.

ok tedu