Create uid/gid convertion function.

Create separate function to convert user ID / group ID of a user to integer.
Windows is a stub, since this doesn't work on Windows (uid/gid is not a number).

lock_unix.go

@@ -0,0 +1,24 @@
+// +build !windows
+
+package restic
+
+import (
+	"os/user"
+	"strconv"
+)
+
+// uidGidInt returns uid, gid of the user as a number.
+func uidGidInt(u user.User) (uid, gid uint32, err error) {
+	var ui, gi int
+	ui, err = strconv.ParseInt(u.Uid, 10, 32)
+	if err != nil {
+		return
+	}
+	gi, err = strconv.ParseInt(u.Gid, 10, 32)
+	if err != nil {
+		return
+	}
+	uid = uint32(ui)
+	gid = uint32(gi)
+	return
+}