Fix "invalid argument" regression.

e2fsprogs v1.43.4 and .5 appear to attempt unaligned writes to disk
devices, which causes "invalid argument" errors and failures.

Patch unix_io.c to always obtain the alignment requirements on
FreeBSD, so that e2fsprogs uses its own write buffers and read-
modify-write cycles when writing only parts of a block.

Bump PORTREVISION, as crucial executables and libs change.

While here, bump self-test requirement to FreeBSD >= 12,
since the maintainer has tested these on 11.0/11.1 amd64/i386.

Reported by: Artyon Mirgorodsky (in private)