hostname-util: add relax parameter to hostname_is_valid

Tests are modified to check behaviour with relax and without relax.
New tests are added for hostname_cleanup().
Tests are moved a new file (test-hostname-util) because there's
now a bunch of them.

New parameter is not used anywhere, except in tests, so there should
be no observable change.

src/basic/util.h

@@ -858,6 +858,11 @@ int unquote_first_word(const char **p, char **ret, UnquoteFlags flags);
 int unquote_first_word_and_warn(const char **p, char **ret, UnquoteFlags flags, const char *unit, const char *filename, unsigned line, const char *rvalue);
 int unquote_many_words(const char **p, UnquoteFlags flags, ...) _sentinel_;
 
+static inline void free_and_replace(char **s, char *v) {
+        free(*s);
+        *s = v;
+}
+
 int free_and_strdup(char **p, const char *s);
 
 #define INOTIFY_EVENT_MAX (sizeof(struct inotify_event) + NAME_MAX + 1)