tree-wide: make cunescape*() functions return ssize_t

Strictly speaking, we are returning the size of a memory chunk of
arbitrary size, so ssize_t is more appropriate than int.

src/nspawn/nspawn.c

@@ -1565,7 +1565,7 @@ static int parse_argv(int argc, char *argv[]) {
                         const char *p = optarg;
                         Credential *a;
                         size_t i;
-                        int l;
+                        ssize_t l;
 
                         r = extract_first_word(&p, &word, ":", EXTRACT_DONT_COALESCE_SEPARATORS);
                         if (r == -ENOMEM)