www/caddy: Change all caddy runtime locations

Thanks to work by scf, the caddy rc(8) script has been completely
rewritten and substantially improved, though this necessitated
changing all runtime locations.

Most importantly:

• The runtime log now lives in /var/log/caddy/ instead of /var/log, so that the logdir can be null-mounted

• Automatic SSL certs are now stored in /var/db/caddy/data/caddy, instead of /root/.local/share/caddy

Caddy uses freedesktop.org's XDG base dir specification, which
conflicts quite spectacularly with FreeBSD's hier(7). The most
sensible thing to do was to put the runtime data stores into
/var/db/caddy, though XDG needs XDG_DATA_HOME and XDG_CONFIG_HOME
to be separate dirs and caddy will create a .../caddy subdir in
each of them.