util: add new helper in_utc_timezone()

As the name suggests it checks whether we are running in an UTC
timezone.

src/basic/time-util.c

@@ -1457,3 +1457,9 @@ usec_t usec_shift_clock(usec_t x, clockid_t from, clockid_t to) {
                 /* x lies in the past */
                 return usec_sub_unsigned(b, usec_sub_unsigned(a, x));
 }
+
+bool in_utc_timezone(void) {
+        tzset();
+
+        return timezone == 0 && daylight == 0;
+}

src/basic/time-util.h

@@ -156,6 +156,8 @@ struct tm *localtime_or_gmtime_r(const time_t *t, struct tm *tm, bool utc);
 
 unsigned long usec_to_jiffies(usec_t usec);
 
+bool in_utc_timezone(void);
+
 static inline usec_t usec_add(usec_t a, usec_t b) {
         usec_t c;