[addrconf] Add applied status to lease

"applied" will be used to denote if lease information has been successfully
installed via update.c.
[netinfo] On new lease, set applied state

FALSE denotes the lease information has not yet been installed to the system
via update.c.
[system_update] Set lease applied state

When lease information is installed to or removed from the system, set
the correct applied state.
[system_update] Remove lease information from system directly

If a lease from which we are to update the system is destined for removal,
do so directly instead (by invoking ni_system_updater_remove()), rather
than going through the _add_source() method.
[system_update] Add concept of max source weight

As we add updater sources (wrappers around leases) to a particular updater,
track the max weight/importance of a source. This will be used to compute
the list of leases whose information we want to install to the system.

Make dhcp4 and dhcp6 the same weight to ensure both get installed via our
extension scripts.
[system_update] Function to compute list of updater sources

When dealing with multiple leases of the same weight attached to an updater,
we may wish to install more than just one. As such, a single "best"
updater_source/lease is not enough. This function computes an array
of updater_source pointers of the highest available weight for an updater.
The system update can then act on these sources.
[system_update] Handle installation of multiple leases

Instead of picking one "best" updater source (and hence lease) to install,
compute an array of updater source points of the max weight. If the leases
they contain are not yet installed, do so.