sbin/hammer: Add full mode for hammer recover to revive full scan

sbin/hammer: Add full mode for hammer recover to revive full scan

This commit revives the original full scan recovery by adding full
option, after the previous commit introduced offset limit.
Apparently, both full option and quick option can't be specified.

To summarize 3 modes,

1. default - Full scan, but only upto the last big-block being used.
2. full - Full scan, which scans the entire fs image with no limit.
3. quick - B-Tree only scan, plus associated records in other zones.

1. was introduced (by the previous commit) to fix a bug, as well as
  to avoid irrelevant files.
2. was introduced (by this commit) to revive the original full scan
  recovery behavior, which is by far the slowest, but most reliable
  in terms of recovery except for the above bug.
3. was introduced (by e819b271) to speed up the recovery process,
  provided B-Tree zone is not corrupted. This is the fastest.