libpam: provide a real /etc/pam.d/ftp

We provide both /etc/pam.d/ftp and /etc/pam.d/ftpd because FTP servers
are inconsistent about which service name they use. Previously ftp was
a hard link to ftpd; to avoid hard linking config files, replace ftp
with a normal config file that includes the ftpd service.

Previously /etc/pam.d/ftpd was in the -ftp package, while /etc/pam.d/ftp
was in the -runtime package (by accident, not design). Move both of
them to the -runtime package, where the rest of the pam.d services live,
since FTP servers from ports use it and people using a ports FTP server
might not want to to install the -ftp package as well.

This also fixes /etc/pam.d/ftp missing the @config tag.

Reviewed by: des, bapt
Approved by: des (mentor)
Differential Revision: https://reviews.freebsd.org/D50339