Split -p into own synopsis

Split -p into own synopsis

Platform-dependent preparation of the filesystem required by the boot loader
only ever creates a new fileystem without instaling using any bootstrap files.

To reflect reality, turn
	# installboot -nvp vnd0
	Using / as root
	would install bootstrap on /dev/rvnd0c
	using first-stage /usr/mdec/biosboot, second-stage /usr/mdec/boot
	would newfs 545c9bdf92aa18f9.i

into
	# ./obj/installboot -nvp vnd0
	would newfs 4db2c0e89e0d3268.i

and error out if -p is combined with -r or stages:
	$ man -hl./installboot.8
	installboot [-nv] [-r root] disk [stage1 [stage2]]
	installboot [-nv] -p disk

Feedback OK millert