mem: add getter for UID/GID

Since mem.Stat().Sys() always returns nil, there's no
way to determine the user or group ID set by Chown().

One example use case is determining whether a file or
directory is writable by the current user.

Signed-off-by: Dominik Menke <dom@digineo.de>

mem/file.go

@@ -102,12 +102,26 @@ func SetUID(f *FileData, uid int) {
 	f.Unlock()
 }
 
+func GetUID(f *FileData) (uid int) {
+	f.Lock()
+	uid = f.uid
+	f.Unlock()
+	return
+}
+
 func SetGID(f *FileData, gid int) {
 	f.Lock()
 	f.gid = gid
 	f.Unlock()
 }
 
+func GetGID(f *FileData) (gid int) {
+	f.Lock()
+	gid = f.gid
+	f.Unlock()
+	return
+}
+
 func GetFileInfo(f *FileData) *FileInfo {
 	return &FileInfo{f}
 }