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.