Add firmware downloading support for Samsung drives

Tested on Samsung SM1625 SSDs.
In pmap_ts_referenced(), when restarting the loop due to pv list
generation changed, do not drop and immediately relock the pv list.

Suggested and reviewed by:	alc
Sponsored by:	The FreeBSD Foundation
Only lock pvh_global_lock read-only for pmap_page_wired_mappings(),
pmap_is_modified() and pmap_is_referenced(), same as it was done for
pmap_ts_referenced().

Consolidate identical code for pmap_is_modified() and
pmap_is_referenced() into helper pmap_page_test_mappings().

Reviewed by:	alc
Tested by:	pho (previous version)
Sponsored by:	The FreeBSD Foundation
r253616 nuked BINMAKE so we need to adapt to the new definition
generate multiple host keys and do that unconditionally
comment out some stale loader configurations.