sync: add semaphores (#5831)

vlib/time/time_nix.c.v

@@ -35,6 +35,7 @@ type time_t voidptr
 
 // in most systems, these are __quad_t, which is an i64
 struct C.timespec {
+mut:
 	tv_sec  i64
 	tv_nsec i64
 }
@@ -78,3 +79,27 @@ pub struct C.timeval {
 	tv_sec  u64
 	tv_usec u64
 }
+
+// return absolute timespec for now()+d
+pub fn (d Duration) timespec() C.timespec {
+	mut ts := C.timespec{}
+	C.clock_gettime(C.CLOCK_REALTIME, &ts)
+	d_sec := d / second
+	d_nsec := d % second
+	ts.tv_sec += d_sec
+	ts.tv_nsec += d_nsec
+	if ts.tv_nsec > second {
+		ts.tv_nsec -= second
+		ts.tv_sec++
+	}
+	return ts
+}
+
+// return timespec of 1970/1/1
+pub fn zero_timespec() C.timespec {
+	ts := C.timespec{
+		tv_sec:  0
+		tv_nsec: 0
+	}
+	return ts
+}