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.c

@@ -2997,7 +2997,7 @@ char* strshorten(char *s, size_t l) {
 
 bool machine_name_is_valid(const char *s) {
 
-        if (!hostname_is_valid(s))
+        if (!hostname_is_valid(s, false))
                 return false;
 
         /* Machine names should be useful hostnames, but also be