sd-event: check clock argument to sd_event_now()

sd-event: check clock argument to sd_event_now()

sd_event_now() is a public function, so we must check all
arguments for validity. Update man page and add tests.

Sample debug message:
Assertion 'IN_SET(clock, CLOCK_REALTIME, CLOCK_REALTIME_ALARM, CLOCK_MONOTONIC, CLOCK_BOOTTIME, CLOCK_BOOTTIME_ALARM)' failed at src/libsystemd/sd-event/sd-event.c:2719, function sd_event_now(). Ignoring.