core: rework unit name validation and manipulation logic

A variety of changes:

- Make sure all our calls distuingish OOM from other errors if OOM is
  not the only error possible.

- Be much stricter when parsing escaped paths, do not accept trailing or
  leading escaped slashes.

- Change unit validation to take a bit mask for allowing plain names,
  instance names or template names or an combination thereof.

- Refuse manipulating invalid unit name

src/sysv-generator/sysv-generator.c

@@ -310,11 +310,13 @@ static int sysv_translate_facility(const char *name, const char *filename, char
          * out whether something is a target or a service alias. */
 
         if (*name == '$') {
-                if (!unit_prefix_is_valid(n))
-                        return -EINVAL;
+                int k;
 
                 /* Facilities starting with $ are most likely targets */
-                r = unit_name_build(n, NULL, ".target");
+                k = unit_name_build(n, NULL, ".target", &r);
+                if (k < 0)
+                        return k;
+
         } else if (streq_ptr(n, filename))
                 /* Names equaling the file name of the services are redundant */
                 return 0;