Introduce the ZFS Boot Environments menu to the loader menu

If the system was booted with ZFS, a new menu item (#7) appears
It contains an autogenerated list of ZFS Boot Environments

This allows the user to switch to an alternate root file system
Use Cases:

• Revert a failed upgrade
• Concurrently run different versions of FreeBSD with common home directory
• Easier integration with the sysadmin/beadm utility

Requested by: many
Reviewed by: dteske
MFC after: 10 days
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Differential Revision: https://reviews.freebsd.org/D3167