unix: unify NsecToTime{spec,val}, fix for times < 1970

All the implementations of NsecToTimespec and NsecToTimeval were the
same other than types. Write a single version that uses
GOARCH/GOOS-specific setTimespec and setTimeval functions to handle the
types.

The logic in NsecToTimespec and NsecToTimeval caused times before 1970
to have a negative usec/nsec. The Linux kernel requires that usec
contain a positive number; for consistency, we do this for both
NsecToTimespec and NsecToTimeval.

Follow CL 30826 which did the same for syscall.

Change-Id: Id6c6f4fef8450251447d1a5b01f35c2a36b5aeb1
Reviewed-on: https://go-review.googlesource.com/73170
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/timestruct.go

@@ -10,6 +10,31 @@ package unix
 // nanoseconds since the Unix epoch.
 func TimespecToNsec(ts Timespec) int64 { return int64(ts.Sec)*1e9 + int64(ts.Nsec) }
 
+// NsecToTimespec takes a number of nanoseconds since the Unix epoch
+// and returns the corresponding Timespec value.
+func NsecToTimespec(nsec int64) Timespec {
+	sec := nsec / 1e9
+	nsec = nsec % 1e9
+	if nsec < 0 {
+		nsec += 1e9
+		sec--
+	}
+	return setTimespec(sec, nsec)
+}
+
 // TimevalToNsec converts a Timeval value into a number of nanoseconds
 // since the Unix epoch.
 func TimevalToNsec(tv Timeval) int64 { return int64(tv.Sec)*1e9 + int64(tv.Usec)*1e3 }
+
+// NsecToTimeval takes a number of nanoseconds since the Unix epoch
+// and returns the corresponding Timeval value.
+func NsecToTimeval(nsec int64) Timeval {
+	nsec += 999 // round up to microsecond
+	usec := nsec % 1e9 / 1e3
+	sec := nsec / 1e9
+	if usec < 0 {
+		usec += 1e6
+		sec--
+	}
+	return setTimeval(sec, usec)
+}