Break the fs package up into smaller parts.

The purpose of this is to make it easier to maintain and eventually to
allow the rclone backends to be re-used in other projects without
having to use the rclone configuration system.

The new code layout is documented in CONTRIBUTING.

fs/config/config_unix.go

@@ -0,0 +1,37 @@
+// Read, write and edit the config file
+// Unix specific functions.
+
+// +build darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package config
+
+import (
+	"os"
+	"os/user"
+	"strconv"
+	"syscall"
+
+	"github.com/ncw/rclone/fs"
+)
+
+// attemptCopyGroups tries to keep the group the same. User will be the one
+// who is currently running this process.
+func attemptCopyGroup(fromPath, toPath string) {
+	info, err := os.Stat(fromPath)
+	if err != nil || info.Sys() == nil {
+		return
+	}
+	if stat, ok := info.Sys().(*syscall.Stat_t); ok {
+		uid := int(stat.Uid)
+		// prefer self over previous owner of file, because it has a higher chance
+		// of success
+		if user, err := user.Current(); err == nil {
+			if tmpUID, err := strconv.Atoi(user.Uid); err == nil {
+				uid = tmpUID
+			}
+		}
+		if err = os.Chown(toPath, uid, int(stat.Gid)); err != nil {
+			fs.Debugf(nil, "Failed to keep previous owner of config file: %v", err)
+		}
+	}
+}