unix: add Getresuid and Getresgid for OpenBSD

Addresses golang/go#60483 for OpenBSD.

This change was successfully tested on amd64 and adjusted accordingly for the other architectures.

Change-Id: Id63cca342d81d2cc5854eb2923ca7e66bfaa91bf
GitHub-Last-Rev: cff11701438edc20bef2961bb73bf9c6ea81b4a1
GitHub-Pull-Request: golang/sys#160
Reviewed-on: https://go-review.googlesource.com/c/sys/+/500655
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

unix/syscall_openbsd.go

@@ -151,6 +151,21 @@ func Getfsstat(buf []Statfs_t, flags int) (n int, err error) {
 	return
 }
 
+//sysnb	getresuid(ruid *_C_int, euid *_C_int, suid *_C_int)
+//sysnb	getresgid(rgid *_C_int, egid *_C_int, sgid *_C_int)
+
+func Getresuid() (ruid, euid, suid int) {
+	var r, e, s _C_int
+	getresuid(&r, &e, &s)
+	return int(r), int(e), int(s)
+}
+
+func Getresgid() (rgid, egid, sgid int) {
+	var r, e, s _C_int
+	getresgid(&r, &e, &s)
+	return int(r), int(e), int(s)
+}
+
 //sys	ioctl(fd int, req uint, arg uintptr) (err error)
 //sys	ioctlPtr(fd int, req uint, arg unsafe.Pointer) (err error) = SYS_IOCTL
 
@@ -338,8 +353,6 @@ func Uname(uname *Utsname) error {
 // getgid
 // getitimer
 // getlogin
-// getresgid
-// getresuid
 // getthrid
 // ktrace
 // lfs_bmapv