udevadm: make use of the new uuid-enabled triggering for "udevadm trigger"

This adds two things:

- A new switch --uuid is added to "udevadm trigger". If specified a
  random UUID is associated with the synthettic uevent and it is printed
  to stdout. It may then be used manually to match up uevents as they
  propagate through the system.

- The UUID logic is now implicitly enabled if "udevadm trigger --settle"
  is used, in order to wait for precisely the uevents we actually
  trigger. Fallback support is kept for pre-4.13 kernels (where the
  requests for trigger uevents with uuids results in EINVAL).

src/basic/hash-funcs.c

@@ -57,6 +57,9 @@ void path_hash_func(const char *q, struct siphash *state) {
 DEFINE_HASH_OPS(path_hash_ops, char, path_hash_func, path_compare);
 DEFINE_HASH_OPS_WITH_KEY_DESTRUCTOR(path_hash_ops_free,
                                     char, path_hash_func, path_compare, free);
+DEFINE_HASH_OPS_FULL(path_hash_ops_free_free,
+                     char, path_hash_func, path_compare, free,
+                     void, free);
 
 void trivial_hash_func(const void *p, struct siphash *state) {
         siphash24_compress(&p, sizeof(p), state);

src/basic/hash-funcs.h

@@ -82,6 +82,7 @@ extern const struct hash_ops string_hash_ops_free_free;
 void path_hash_func(const char *p, struct siphash *state);
 extern const struct hash_ops path_hash_ops;
 extern const struct hash_ops path_hash_ops_free;
+extern const struct hash_ops path_hash_ops_free_free;
 
 /* This will compare the passed pointers directly, and will not dereference them. This is hence not useful for strings
  * or suchlike. */