Add zfsboot to the scripted interface of bsdinstall(8); oops!
NB: If the zfsboot variables ($ZFSBOOT_*) are set, a script is
assumed to want zfsboot module instead of scriptedpart module.

Submitted by:	Loïc Brarda <loic.brarda@cern.ch>
Reviewed by:	nwhitehorn@
MFC after:	3 days