libpam: Move to a new "pam" package

OpenPAM is a discrete, largely self-contained system component.
Users may not need PAM for many use-cases (e.g. jails, containers),
so move it to its own package.

Use LIB_PACKAGE to create a separate pam-lib package for libpam,
so that applications that support PAM don't need to bring in all
the PAM modules if PAM isn't actually in use.

Add pam to the minimal sets, since this is a core system component that
people expect to be installed. This means all supported installation
methods will install the PAM modules by default, so don't add explicit
dependencies on the PAM modules from things that use PAM (e.g. runtime),
allowing custom/embedded systems to omit these easily.

This change adds a new package to the system so, until we have a proper
policy on how to handle this in release/stable branches, it should not
be MFC'd.

MFC after: never
Reviewed by: des, bapt
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53602