Minor improvements.

Most noticeable changes in no particular order:

• The rc.d script doesn't use the provided shell script to start the daemon, but starts it directly.
• This is now installed in ${DATADIR}, not in ${PREFIX}/${PORTDIRNAME}.
• Define CONFLICTS_INSTALL so as to avoid colliding with non standalone version.
• Use post-install-${OPTION}-on.

PR: 210608
Submitted by: joshruehlig@gmail.com
Reviewed by: crees
Approved by: crees
Differential Revision: https://reviews.freebsd.org/D7304