For configurations where another web server is running on port 80, running
acme.sh in standalone mode fails. Try to detect this and refuse to run; and
allow the user to configure a webroot directory to use the running webserver for
certificate verification.

This also updates acme.sh to the latest version.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

This adds a Luci configuration app for the ACME certificate package.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>

This adds a package wrapping the acme.sh script from
https://github.com/Neilpang/acme.sh in Uci config and hooks to interact
correctly with uhttpd.

Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>