Add 3 long options for getting information about mkimg itself:
--version	print the version of mkimg and also whether it's
		64- or 32-bit.
--formats	list the supported output formats separated by space.
--schemes	list the supported partitioning schemes separated by
		space.

Inspired by a patch from: gjb@

MFC after:	1 week
Relnotes:	yes


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272198 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f