Make pure-ftpd flags configurable in /etc/rc.conf

Recognize a "pureftpd_flags" variable in pure-ftpd's rc script. Pretty much
anything that can be placed in /usr/local/etc/pure-ftpd.conf can be placed
in this variable instead.

ftp/pure-ftpd/Makefile
Bump PORTREVISION

ftp/pure-ftpd/files/pure-ftpd.in
Handle pureftpd_flags. It needs special handling because, unlike
most daemons, pure-config.pl takes its mandatory argument first and
its optional flags last.

PR: 210311
Approved by: maintainer-timeout
Approved by: brd (ports)
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6676