[leasefile] lease to xml WIP

First phase of lease<-->xml

Mostly contains generic lease data. dhcp4 related stuff is derived
via dhcp-lease.c which could stand a re-write to update some
of the API's in use (ie. replace inet_*() calls with ni_sockaddr_*()
variants).

Routes are derived by duplicating code in leaseinfo.c. This is WIP
and so I'll either expose the private routing functions located in
leaseinfo.c, or move them to something like util.c or rework how
route data is stored in xml (ie. perhaps extract each element
seperately rather than route=dest,netmask/prefixlen,gateway).

dhcp6 stuff to come.
[leasefile] print only non-empty strings
[leasefile] dhcp6 WIP