ProFTPd port does not install docs for optional modules

The ftp/proftpd port creates /usr/local/share/doc/proftpd and
installs the two HTML files from ProFTPd's docs directory
into it. But it doesn't copy the html files from the contrib
directory which describe the optional modules (mod_tls,
mod_radius, etc.) and their configuration options.

Also moved the portdocs to the PORTDOCS environment variable.

PR: ports/56511
Submitted by: Pat Lashley <patl+freebsd@volant.org>
Reviewed by: maintainer timeout