dnsmasq: pass environment variables to hotplug

dnsmasq: pass environment variables to hotplug

dnsmasq passes a limited amount of information via DHCP script arguments. Much
more information is available through environment variables starting with
DNSMASQ_, such as DNSMASQ_INTERFACE. However, when the dhcp-script builds its
JSON environment and passes it to hotplug, all of this information is discarded
since it is not copied to the JSON environment.

Personally, I have a custom-made set of DDNS scripts and rely on environment
variables such as DNSMASQ_INTERFACE in order to determine which DNS zones
to update. So, not being able to access these variables was detrimental to me.
I patched in a quick copy of all DNSMASQ_ variables to the JSON environment
so that they can be used in hotplug scripts. In order to do so I also copied
/usr/bin/env into dnsmasq's chroot jail.

Signed-off-by: Chuck R <github@chuck.cloud>
Link: https://github.com/openwrt/openwrt/pull/16354
Signed-off-by: John Crispin <john@phrozen.org>