parse-util: add explicit parsers for MTU values

We use MTUs all over the place, let's add a unified, strict parser for
it, that takes MTU ranges into account.

We already have parse_ifindex() close-by, hence this appears to be a
natural addition, in particular as the range checking is not entirely
trivial to do, as it depends on the protocol used.

src/basic/parse-util.h

@@ -22,6 +22,7 @@ int parse_dev(const char *s, dev_t *ret);
 int parse_pid(const char *s, pid_t* ret_pid);
 int parse_mode(const char *s, mode_t *ret);
 int parse_ifindex(const char *s, int *ret);
+int parse_mtu(int family, const char *s, uint32_t *ret);
 
 int parse_size(const char *t, uint64_t base, uint64_t *size);
 int parse_range(const char *t, unsigned *lower, unsigned *upper);