unix: add new setfsuid/setfsgid wrappers

Added SetfsgidRetGid and SetfsuidRetUid wrappers for corresponding
syscalls, both returning previous gid or uid set.

Fixes golang/go#36649

Change-Id: I95176f4442258ca68db79cf967fbf0bbf0a57acf
Reviewed-on: https://go-review.googlesource.com/c/sys/+/215579
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/syscall_linux.go

@@ -1654,6 +1654,30 @@ func Setgid(uid int) (err error) {
 	return EOPNOTSUPP
 }
 
+// SetfsgidRetGid sets fsgid for current thread and returns previous fsgid set.
+// setfsgid(2) will return a non-nil error only if its caller lacks CAP_SETUID capability.
+// If the call fails due to other reasons, current fsgid will be returned.
+func SetfsgidRetGid(gid int) (int, error) {
+	return setfsgid(gid)
+}
+
+// SetfsuidRetUid sets fsuid for current thread and returns previous fsuid set.
+// setfsgid(2) will return a non-nil error only if its caller lacks CAP_SETUID capability
+// If the call fails due to other reasons, current fsuid will be returned.
+func SetfsuidRetUid(uid int) (int, error) {
+	return setfsuid(uid)
+}
+
+func Setfsgid(gid int) error {
+	_, err := setfsgid(gid)
+	return err
+}
+
+func Setfsuid(uid int) error {
+	_, err := setfsuid(uid)
+	return err
+}
+
 func Signalfd(fd int, sigmask *Sigset_t, flags int) (newfd int, err error) {
 	return signalfd(fd, sigmask, _C__NSIG/8, flags)
 }