HEADS UP (16.0 pkgbase): new package for PAM

if you don't use pkgbase, you can ignore this mail.

hello,

i am about to commit a change to main (16.0) which adds a new package
for OpenPAM.  there are two notable packages in this change:

* FreeBSD-pam-lib contains libpam, which is used by applications that
  want to authenticate users (e.g. login(1) or sshd(8)), and will be
  installed automatically via shlib deps in most cases.

* FreeBSD-pam contains the PAM modules (e.g., pam_unix.so), which are
  required for PAM user authentication to work; this package will *not*
  be installed via shlib deps.

FreeBSD-pam will be automatically installed if the minimal(-jail) set is
installed, or if the optional or base sets are installed, since those
depend on minimal.  if you don't have any of those sets installed, and
you need user authentication, you MUST install the FreeBSD-pam package
when updating past this change or you will no longer be able to log in.

(if you don't have set-minimal(-jail) installed, i recommend installing
it since essential system components will always be part of the minimal
set, but this isn't a requirement if you prefer to install new packages
manually.)

this change will not be MFC'd to stable/15.