Remove all cross references to zfsloader.8 and /boot/zfsloader.
Move ZFS specific info into loader.8.
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D16361
zfsloader(8) fails to probe a slice containing ZFS pool if its second sector
contains traces of BSD label (DISKMAGIC == 0x82564557).
Fix manual page to show working example erasing such traces.
PR: 226714
Approved by: avg (mentor)
MFC after: 3 days