PAM module for loading ZFS keys on login

[Eric McCorkle]

All,

This patch creates a new PAM module that will load a ZFS key upon a
successful login: https://reviews.freebsd.org/D31844.  It will use the
user's auth token as the key argument to loading a ZFS encryption key on
a user-specific ZFS data set.

This is the other side of my changeset to have autounmountd unload ZFS
keys when it unloads a filesystem.  (Here:
https://reviews.freebsd.org/D31725)  With these two changes, it should
be possible to have ZFS encrypted home directories with keys dynamically
loaded when users log in, and unloaded when their home directories are
unmounted.

Please review and comment.