sbin/hammer: Add quick mode for hammer recover

sbin/hammer: Add quick mode for hammer recover

Since hammer recover command tries to recover filesystem data
based on assumption on ondisk data bytes that look like B-Tree
nodes/elms, the command can tell the recovery process is done
once scanning offset gets to the point where there is no more
big-blocks for B-Tree zone, without scanning through the whole
address space of all volumes (provided B-Tree zone is alive).

By specifying quick option after the target directory option,
this command makes use of B-Tree big-block info prefetched before
recovery process, and stops recovery once all B-Tree big-blocks
are scanned. As shown in below example, this makes recovery
much faster by cutting unnecessary I/Os.

The drawback is that quick mode is based on assumption that
B-Tree zone isn't corrupted. If B-Tree zone is somehow corrupted,
prefetched info is incomplete or totally wrong, so one needs
to linearly scan the whole address space of all volumes to
check every possible B-Tree nodes/elms without using quick mode
which is what's been done by default.

-- example of default and quick mode
 # newfs_hammer -L TEST /dev/da1 > /dev/null
 # mount_hammer /dev/da1 /HAMMER
 # cd /HAMMER
 # git clone /usr/local/src/dragonfly > /dev/null 2>&1
 # cd
 # umount /HAMMER

 # time hammer -f /dev/da1 recover /tmp/a > /dev/null
 hammer -f /dev/da1 recover /tmp/a > /dev/null  309.51s user 122.96s system 21% cpu 33:50.17 total
 # cd /tmp/a/PFS00000/dragonfly/sys/vfs/hammer
 # make > /dev/null 2>&1; echo $?
 0
 # file hammer.ko
 hammer.ko: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped

 # time hammer -f /dev/da1 recover /tmp/b quick > /dev/null
 hammer -f /dev/da1 recover /tmp/b quick > /dev/null  0.41s user 3.41s system 14% cpu 26.652 total
 # cd /tmp/b/PFS00000/dragonfly/sys/vfs/hammer
 # make > /dev/null 2>&1; echo $?
 0
 # file hammer.ko
 hammer.ko: ELF 64-bit LSB relocatable, x86-64, version 1 (SYSV), not stripped