bind plugin: Use timegm() to convert to time_t if available.

bind plugin: Use timegm() to convert to time_t if available.

FreeBSD doesn't provide the "extern long timezone", presumably because
it's "only" an XSI extension, leading to a portability issue with the
previous approach.

timegm() is a non-standard function available in GNU's and BSD's libc
which is doing exactly what we need. The previous code is left as a
fallback.

Fixes: #1268