tree-wide: introduce mfree()

Pretty trivial helper which wraps free() but returns NULL, so we can
simplify this:
        free(foobar);
        foobar = NULL;
to this:
        foobar = mfree(foobar);

src/shared/conf-parser.c

@@ -333,8 +333,7 @@ int config_parse(const char *unit,
                                 return -ENOMEM;
                         }
 
-                        free(continuation);
-                        continuation = NULL;
+                        continuation = mfree(continuation);
                         p = c;
                 } else
                         p = l;