Apply detected capabilities and disable DHCP if not capable.

[dschaper]

s6-setuidgid changes users and the new docker binary drops all capabilities on user change. Revert the revert to use capsh again to push caps to Ambient.

Detect if NET_ADMIN is not available and DHCP_ACTIVE=true. Warn user that DHCP will not be started but continue in DNS only mode.

Tested with the following runs on Debian Buster:

  docker run -it --rm --name=pitest --network=host -e FTLCONF_DEBUG_CAPS=true -e DHCP_ACTIVE=true -e DHCP_START=8.8.8.8 -e DHCP_END=9.9.9.9 -e DHCP_ROUTER=12.12.12.12 djschaper/pihole-caps-test:new
  docker run -it --rm --name=pitest --network=host -e FTLCONF_DEBUG_CAPS=true  djschaper/pihole-caps-test:new
  docker run -it --rm --name=pitest  -e FTLCONF_DEBUG_CAPS=true -e DHCP_ACTIVE=true -e DHCP_START=8.8.8.8 -e DHCP_END=9.9.9.9 -e DHCP_ROUTER=12.12.12.12 djschaper/pihole-caps-test:new
  docker run -it --rm --name=pitest djschaper/pihole-caps-test:new

DHCP && host network
DHCP && bridge network
NO DHCP && host network
NO DHCP && bridge network

All good.