time-util: add new call usec_shift_clock() for converting times between clocks

We use that quite often, let's implement one clean version of it.

src/basic/time-util.h

@@ -145,6 +145,8 @@ bool clock_boottime_supported(void);
 bool clock_supported(clockid_t clock);
 clockid_t clock_boottime_or_monotonic(void);
 
+usec_t usec_shift_clock(usec_t, clockid_t from, clockid_t to);
+
 #define xstrftime(buf, fmt, tm) \
         assert_message_se(strftime(buf, ELEMENTSOF(buf), fmt, tm) > 0, \
                           "xstrftime: " #buf "[] must be big enough")