Enable the creation of lfs64 volumes.

To create an lfs64 volume, use -w 64. You can also force a 32-bit
volume with -w 32, but this will fail on devices thta are too large.

lfs64 is the default for devices > 1TB. (1TB rather than 2TB because
daddr_t is signed and negative block numbers can cause interesting
complications.)

For now print a warning that the lfs64 format isn't finalized, because
it isn't. For now any lfs64 use should be limited to test data,
benchmarking, and so forth.

(dholland)