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

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

src/test/test-util.c

@@ -34,6 +34,7 @@
 #include "conf-parser.h"
 #include "cpu-set-util.h"
 #include "def.h"
+#include "escape.h"
 #include "fileio.h"
 #include "mkdir.h"
 #include "process-util.h"