Avoid using labs() on time_t in channeltls.c

On some windows builds, time_t is 64 bits but long is not.  This is
causing appveyor builds to fail.

Also, one of our uses of labs() on time_t was logically incorrect:
it was telling us to accept NETINFO cells up to three minutes
_before_ the message they were responding to, which doesn't make
sense.

This patch adds a time_abs() function that we should eventually move
to intmath.h or something.  For now, though, it will make merges
easier to have it file-local in channeltls.c.

Fixes bug 31343; bugfix on 0.2.4.4-alpha.