Generate code that uses the `Ipaddr.V4` interface instead of `Nettypes`.

This means that Mirari now needs Mirage 0.9.5 and Mirage-ent 0.9.4
Replace opam by mirari in the help message. Fix #21.
XL configuration file is now created during configuration phase.
Output tuntap backend only when direct is selected.
Introduce preliminary support for the kFreeBSD backend.
[kfreebsd] Generate moduledata_t for the kernel modules.
Add a changelog entry on the kFreeBSD backend support.