resolved: apply epoch to system time from PID 1

For use in timesyncd we already defined a compile-time "epoch" value, which is based on the mtime of the NEWS file, and
specifies a point in time we know lies in the past at runtime. timesyncd uses this to filter out nonsensical timestamp
file data, and bump the system clock to a time that is after the build time of systemd. This patch adds similar bumping
code to earliest PID 1 initialization, so that the system never continues operation with a clock that is in the 1970ies
or even 1930s.

src/basic/clock-util.h

@@ -28,3 +28,4 @@ int clock_set_timezone(int *min);
 int clock_reset_timewarp(void);
 int clock_get_hwclock(struct tm *tm);
 int clock_set_hwclock(const struct tm *tm);
+int clock_apply_epoch(void);