util: split out escaping code into escape.[ch]

This really deserves its own file, given how much code this is now.

src/nspawn/nspawn-mount.c

@@ -19,17 +19,18 @@
   along with systemd; If not, see <http://www.gnu.org/licenses/>.
 ***/
 
-#include <sys/mount.h>
 #include <linux/magic.h>
+#include <sys/mount.h>
 
-#include "util.h"
-#include "rm-rf.h"
-#include "strv.h"
-#include "path-util.h"
-#include "mkdir.h"
-#include "label.h"
-#include "set.h"
 #include "cgroup-util.h"
+#include "escape.h"
+#include "label.h"
+#include "mkdir.h"
+#include "path-util.h"
+#include "rm-rf.h"
+#include "set.h"
+#include "strv.h"
+#include "util.h"
 
 #include "nspawn-mount.h"