MFT: For staging, still run install-mtree. This fixes orphans/leftovers when testing ports-mgmt/pkg.

pkg_install: Because this uses pkg_add(1), it is properly installing the MTREE,
             but then on deinstall it leaves all of /usr/local behind. These
	     were seen as leftovers.
pkg:         pkg register -i does not support MTREE, so it never was creating
             the /usr/local MTREE, resulting in orphans

Reported by:	madpilot
MFT: Add warnings about using csup and clean up the language around it.