Minor mdoc improvements.

Approved by:	re (blanket)
Remove extra 'MAKE_FLAGS' line.

Submitted by:	jhb
Approved by:	re (marius)
Sponsored by:	The FreeBSD Foundation
Further refinement to bmake bootstrapping

Include PROGNAME and DESTDIR in ${MMAKE} so that it doesn't need to be
passed to each make invocation.

Suggested by:	hrs
Reviewed by:	hrs
Approved by:	re (gjb)