util: add creds-util.[ch] with helpers for dealing with credentials

src/basic/path-util.c

@@ -1190,9 +1190,3 @@ bool prefixed_path_strv_contains(char **l, const char *path) {
 
         return false;
 }
-
-bool credential_name_valid(const char *s) {
-        /* We want that credential names are both valid in filenames (since that's our primary way to pass
-         * them around) and as fdnames (which is how we might want to pass them around eventually) */
-        return filename_is_valid(s) && fdname_is_valid(s);
-}