Merge branch 'mem-pool-range'

Merge branch 'mem-pool-range'

Adds support to configure address pools as ranges (from-to) in
ipsec.conf and swanctl.conf.

The first and last addresses in subnet based pools are now skipped
properly and the pools' sizes are adjusted accordingly.  Which is also
the case if pools are configured with an offset, e.g. 192.168.0.100/24,
which reduces the number of available addresses from 254 to 155, and
assignment now starts at .100 not .101, i.e. .100-.254 are assignable
to clients.

References #744.