tree-wide: fall back to now(CLOCK_MONOTONIC) if CLOCK_BOOTTIME unsupported (#3037)

It was added in 2.6.39, and causes an assertion to fail when running in mock
hosted on 2.6.32-based RHEL-6:

Assertion 'clock_gettime(map_clock_id(clock_id), &ts) == 0' failed at systemd/src/basic/time-util.c:70, function now(). Aborting.

src/core/timer.c

@@ -320,7 +320,7 @@ static usec_t monotonic_to_boottime(usec_t t) {
         if (t <= 0)
                 return 0;
 
-        a = now(CLOCK_BOOTTIME);
+        a = now(clock_boottime_or_monotonic());
         b = now(CLOCK_MONOTONIC);
 
         if (t + a > b)