Refactor isc_time_now() to return time, and not result

The isc_time_now() and isc_time_now_hires() were used inconsistently through the code - either with status check, or without status check, or via TIME_NOW() macro with RUNTIME_CHECK() on failure.

Refactor the isc_time_now() and isc_time_now_hires() to always fail when getting current time has failed, and return the isc_time_t value as return value instead of passing the pointer to result in the argument.