MFC r304850, r305480, r324550-r324551, r324655, r324684: correct mis-merge

Some of these commits were improperly MFC'd in the sys/boot => stand
mega-MFC, others were simply missed. Correct that mistake now by manually
merging the few that were missed and record-only merge on the others.

r304850:
Unused variables and cstyle fix for loader dosfs

r305480:
Renumber the advertising clause.

r324550:
Add $FreeBSD$ to ancient sources that it's missing from.

r324551:
Move lib/libstand to sys/boot/libsa

Move the sources to sys/boot. Make adjustments related to the
move. Kill LIBSTAND_SRC since it's no longer needed.

r324655:
Remove the libstand directory which is now empty.

r324684:
Remove lib/libstand again, accidentally readded in r324683