iv_time_posix: Use CLOCK_MONOTONIC_FAST if available.

Since CLOCK_MONOTONIC is excruciatingly slow on FreeBSD, and while
CLOCK_MONOTONIC_FAST is faster but less precise, we don't need the
extra precision for iv_now as much as we need the extra speed.

Suggestion from Balazs Scheidler <bazsi@balabit.hu>.

Signed-off-by: Lennert Buytenhek <buytenh@wantstofly.org>