keep file permissions and try to keep user/group on supported systems (fixes #1467)

fs/config_other.go

@@ -0,0 +1,10 @@
+// Read, write and edit the config file
+// Non-unix specific functions.
+
+// +build !darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd,!solaris
+
+package fs
+
+// attemptCopyGroups tries to keep the group the same, which only makes sense
+// for system with user-group-world permission model.
+func attemptCopyGroup(fromPath, toPath string) {}