Document IPV4 module type and fix a number of inconsistencies.

* ipaddr -> ipv4addr to match udp/tcp
* Add missing gateway accessor
* Clarify ip to ipv4 in function names
Merge branch 'split-libs' of github.com:avsm/mirage into split-libs
Document UDPV4 module types
Document TCPv4
More docs for CHANNEL, and remove the unused Closed exception
Merge pull request #206 from avsm/split-libs

Doc and some inconsistency fixing in types