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/basic/time-util.h

@@ -112,6 +112,7 @@ bool ntp_synced(void);
 int get_timezones(char ***l);
 bool timezone_is_valid(const char *name);
 
+bool clock_boottime_supported(void);
 clockid_t clock_boottime_or_monotonic(void);
 
 #define xstrftime(buf, fmt, tm) \