os/user: wrap getgrnam_r to fix type issues

Even with -D_POSIX_PTHREAD_SEMANTICS, Solaris seems to not define
getgrnam_r in a POSIX compatible way.

Fixes #14967

Change-Id: I78cb7e5b30b2d8b860e336060a0a06f4720c0475
Reviewed-on: https://go-review.googlesource.com/21385Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/os/user/lookup_unix.go

@@ -37,6 +37,11 @@ static int mygetgrgid_r(int gid, struct group *grp,
 	char *buf, size_t buflen, struct group **result) {
  return getgrgid_r(gid, grp, buf, buflen, result);
 }
+
+static int mygetgrnam_r(const char *name, struct group *grp,
+	char *buf, size_t buflen, struct group **result) {
+ return getgrnam_r(name, grp, buf, buflen, result);
+}
 */
 import "C"
 
@@ -139,7 +144,7 @@ func lookupGroup(groupname string) (*Group, error) {
 	defer C.free(unsafe.Pointer(cname))
 
 	err := retryWithBuffer(buf, func() syscall.Errno {
-		return syscall.Errno(C.getgrnam_r(cname,
+		return syscall.Errno(C.mygetgrnam_r(cname,
 			&grp,
 			(*C.char)(buf.ptr),
 			C.size_t(buf.size),