dgram: make setMulticastTTL() conform to v0.4 API

- throw if the ttl argument is not a number
- return the ttl argument (not particulary useful but it's what v0.4 did)

Note that the 0 < ttl < 256 check has *not* been reinstated. On Linux, -1 is a
valid argument to setsockopt(IPPROTO_IP, IP_TTL).