dnsmasq: add host-specific lease time option for static hosts

dnsmasq: add host-specific lease time option for static hosts

Enable setting a host-specific lease time for static hosts.
The new option is called "leasetime" and the format is similar
as for the default lease time: e.g. 12h, 3d, infinite

Default lease time is used for all hosts for which there is
no host-specific definition.

The option is added to /etc/config/dhcp for the selected hosts:
  config host
        option name 'Nexus'
        option mac 'd8:50:66:55:59:7c'
        option ip '192.168.1.245'
        option leasetime '2h'

It gets appended to /var/etc/dnsmasq.conf like this:
  dhcp-host=d8:50:66:55:59:7c,192.168.1.245,Nexus,2h

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48801 3c298f89-4303-0410-b956-a3cf2f4a3e73