add gptzfsboot.8, zfsboot.8 and zfsloader.8 manual pages

Many thanks to Warren Block for his reviews, corrections and additions.

Reviewed by:	Warren Block <wblock@FreeBSD.org>
MFC after:	1 week
Make net.inet.ip.sourceroute, net.inet.ip.accept_sourceroute, and
net.inet.ip.process_options vnet-aware.  Revert changes in r271545.

Suggested by:	bz
Remove incorrect props