www/xcaddy: Add rc(8) script

Bring in the rc(8) script from caddy to xcaddy so that xcaddy users
can control caddy like an adult.

Add CONFLICTS between xcaddy and caddy because they now install the
same rc(8) script. There's no reason to change the install path here
because, as olgeni put it, "xcaddy with plugins is more caddy than
caddy".

Suggested by: bdrewery
Approved by: olgeni (xcaddy maintainer)