add hostsfile output in addition to statefile

add hostsfile output in addition to statefile

a92c0a7 made the temporary state/leasefile hidden so that an atomic
change was made and dnsmasq only saw the new file on rename.  A
misguided optimisation was made to only rename the temporary file if
something had changed.  Unfortunately only address and hostnames were
considered in the change, lease durations were not.

As a result it was possible for LUCI which consumes the state/leasefile
to report DHCPv6 leases had expired when they had not.

Revert the optimisation so that the file rename occurs irrespective of
content change, this keeps LUCI reporting of state/lease expiry correct.

This leaves us back with hosts file/dnsmasq update problem. Solve this
by writing out a separate hosts file.  Update this file using the
original IP/Hostname change logic that prompts calling the 'lease'
script.

odhcpd config now supports a string 'hostsfile' which defines the path
and name of the hosts file in an identical manner to 'leasefile'.  A
state 'leasefile' must be defined IF a 'hostsfile' is also required.

eg.

leasefile '/tmp/odhcpdstate'
hostsfile '/tmp/hosts/odhcpdhosts'

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>