Fix bounced I/O on unaligned buffers (FreeBSD 11+)

The bounce-buffer I/O driver fix in patch-lib_ext2fs_unix__io.c is a
prototypical fix, and has been forwarded upstream, but it permits us to
pass all self-tests (mostly undo-related) that were still failing on
11.0-RC2 amd64. There is some potential for optimization,
posix_memalign() might save us quite a few memcpy() calls and replace
the now-repaired code paths by a simple pread[64]() or pwrite[64]().

Remove BROKEN, but insist (by setting BROKEN conditionally) on anything
that is FreeBSD 11 or newer, or non-i386/non-amd64, that the user runs
at least the small self-tests.

Revise option descriptions a bit.

Bump PORTREVISION.