x/sys/unix: allow nil on unix for all variants of Utimes

Fixes golang/go#11830.

Change-Id: Ie6e9f82d05b7c04092b6ee6117238873b746380e
Reviewed-on: https://go-review.googlesource.com/12690
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_solaris.go

@@ -270,7 +270,10 @@ func Gethostname() (name string, err error) {
 	return name, err
 }
 
-func UtimesNano(path string, ts []Timespec) (err error) {
+func UtimesNano(path string, ts []Timespec) error {
+	if ts == nil {
+		return Utimes(path, nil)
+	}
 	if len(ts) != 2 {
 		return EINVAL
 	}