sprinkle const in ffs1_blkpref(), ffs2_blkpref()

make ffs1_blkpref() and ffs2_blkpref() receive a const pointer to the
array of block pointers to make it clear that they are not modified
within those functions.

while there, remove pointless assignment from ffs1_blkpref() and make
the pointer to the superblock a const in both functions.

OK oga@
add const to ffs block verification functions

add const to ffs_checkblk(), ffs_isfreeblock(), ffs_isblock(),
ffs_clrblock() and ffs_setblock(); hopefully a trivial change.

OK oga@
add const to ffs allocation functions

sprinkle const throughout most of ffs_alloc.c and ffs_balloc.c, in the
hope of making the code easier to reason about; adapt softdep
accordingly.

OK oga@
add const to ffs deallocation functions

add const to the block deallocation bits of ffs_alloc.c

OK oga@
add quirk for armv7/sunxi ahci, this is the 'less MD' flavour of the
diff using a flag in softc, instead of at compile-time with #ifdef.
fix ufs_ihashrem() use of LIST fields

fix pointless (and potentially dangerous) manipulation of LIST fields in
ufs_ihashrem().

OK oga@
remove b_synctime

only set and never used, it was doing nothing but eating 8 bytes from
every buffer.

OK natano@
Nit-pick bsd.port.mk(5).

Remove statement that CONFIGURE_STYLE = simple together with
MODULES += gnu produces the effects of MODGNU_CONFIG_GUESS_DIRS, as it
is not true.

ok haesbaert@