dnsmasq: Allow creating static DNS entries from static lease entries

DHCP entries in /etc/config/dhcp will not automatically create A or PTR
records.  Add an "option dns" directive which appends an entry to
/tmp/hosts/dhcp to facilitate forward and reverse DNS lookups.  For
instance, this item:

    config host
            option ip       '192.168.0.10'
            option mac      '00:13:57:9b:df:02'
            option name     'winpc'
            option dns      '1'

will add a corresponding entry to /tmp/hosts/dhcp:

    192.168.0.10 winpc.lan

This keeps the hostname/IP/MAC in a single place, for easy maintenance.

Related: ticket #13854 reports an regression involving missing PTR
records when using "config domain" to define static DNS entries for
individual hosts.  However, per Simon Kelley[1], the --address feature
used by "config domain" was never intended to generate DNS A records for
hosts.  It would probably be better for the reporter to apply this patch,
and then use "config host" sections instead of "config domain" sections.

[1] http://lists.thekelleys.org.uk/pipermail/dnsmasq-discuss/2008q4/002498.html

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Signed-off-by: Florian Fainelli <florian@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@40799 3c298f89-4303-0410-b956-a3cf2f4a3e73