Augment STACKV4 with an IPV4 module in addition to TCPV4 and UDPV4.

This should really have been in the original commit, since without
the IPV4 module exposed, it's not possible to manipulate the IP
configuration of the stack from an external API.

This requires the associated commit in mirage/mirage-tcpip#50
Merge branch 'master' of github.com:mirage/mirage
update CHANGES
Regenerate with OASIS 0.4.4 (which adds natdynlink support)
Merge pull request #239 from avsm/master

Augment STACKV4 with an IPV4 module in addition to TCPV4 and UDPV4