tree-wide: don't assume CLOCK_BOOTIME is generally available

Before we invoke now(CLOCK_BOOTTIME), let's make sure we actually have that
clock, since now() will otherwise hit an assert.

Specifically, let's refuse CLOCK_BOOTTIME early in sd-event if the kernel
doesn't actually support it.

This is a follow-up for #3037, and specifically:

https://github.com/systemd/systemd/pull/3037#issuecomment-210199167

src/core/timer.c

@@ -373,7 +373,7 @@ static void timer_enter_waiting(Timer *t, bool initial) {
          * rather than the monotonic clock. */
 
         ts_realtime = now(CLOCK_REALTIME);
-        ts_monotonic = now(t->wake_system ? CLOCK_BOOTTIME : CLOCK_MONOTONIC);
+        ts_monotonic = now(t->wake_system ? clock_boottime_or_monotonic() : CLOCK_MONOTONIC);
         t->next_elapse_monotonic_or_boottime = t->next_elapse_realtime = 0;
 
         LIST_FOREACH(value, v, t->values) {