Timestamp: fix integer overflow in freeze_timestamp()

When converting the value of mach_absolute_time() into milliseconds,
multiplying a uint64_t by an int was producing wrong results on big-
endian architectures (e.g. PowerPC) due to the larger value of
s_timebase_info.denom on that platform. This resulted in implausible
timestamps that caused mosh to exit instantly on starting up.

Fixes #479.