fsck_msdosfs: add tests for lost cluster chain repair accountingHEADmain

Add an ATF test suite covering Phase 3 ("Checking for Lost Files")
error accounting.  Test images are created using newfs_msdos(8),
and lost cluster chains are injected directly into FAT copies at
offsets derived from the BPB.  The LOST.DIR directory required by
reconnect() is constructed similarly: a root directory entry with
ATTR_DIRECTORY set and its first cluster pointing to a zero-filled
cluster containing "." and ".." entries.

The lost_chain_cleared and corrupted_lost_chain_reconnected test
cases provide regression coverage for the preceding commit:
 - lost_chain_cleared verifies that clearing a lost chain (the fallback
   taken when LOST.DIR is absent) exits with status 0 rather than 8
   (unrecovered error).
 - corrupted_lost_chain_reconnected verifies that FAT modifications
   from a chain truncated by checkchain() prior to reconnection are
   written back to disk, requiring "Update FATs? yes" and ensuring a
   clean second pass.

Additionally, lost_chain_left_alone, lost_chain_preen, and
corrupted_lost_chain_left_alone cover scenarios that must continue to
report unrecovered errors: read-only mode (-n), which performs no
repairs and leaves the image byte-for-byte unchanged, and preen mode
(-p), which attempts reconnection but does not clear lost chains.

MFC after:      1 week

0 files changed, 0 insertions, 0 deletions