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
Perhaps RB_MUTE could mute user startup (rc) output as well, but right
now it mutes only kernel console output, so make the documentation match
reality.
PR: 228193
Sponsored by: The FreeBSD Foundation