Rewrite the disk I/O management system in fsck_ffs(8). Other than
making fsck_ffs(8) run faster, there should be no functional change.

The original fsck_ffs(8) had its own disk I/O management system.
When gjournal(8) was added to FreeBSD 7, code was added to fsck_ffs(8)
to do the necessary gjournal rollback. Rather than use the existing
fsck_ffs(8) disk I/O system, it wrote its own from scratch. Similarly
when journalled soft updates were added in FreeBSD 9, code was added
to fsck_ffs(8) to do the necessary journal rollback. And once again,
rather than using either of the existing fsck_ffs(8) disk I/O
systems, it wrote its own from scratch. Lastly the fsdb(8) utility
uses the fsck_ffs(8) disk I/O management system. In preparation for
making the changes necessary to enable snapshots to be taken when
using journalled soft updates, it was necessary to have a single
disk I/O system used by all the various subsystems in fsck_ffs(8).

This commit merges the functionality required by all the different
subsystems into a single disk I/O system that supports all of their
needs. In so doing it picks up optimizations from each of them
with the results that each of the subsystems does fewer reads and
writes than it did with its own customized I/O system. It also
greatly simplifies making changes to fsck_ffs(8) since everything
goes through a single place. For example the ginode() function
fetches an inode from the disk. When inode check hashes were added,
they previously had to be checked in the code implementing inode
fetch in each of the three different disk I/O systems. Now they
need only be checked in ginode().

Tested by: Peter Holm
Sponsored by: Netflix