t: Reverted most of LFS_T_MKCONSISTENT, just check for new grms/orphans

Checking for orphans + other traversal work turned out to mesh much
worse than originally thought:

- Adjusting mids and being able to drop mdirs mid-traversal complicates
  traversal quite a bit and has potential to hide difficult to reproduce
  bugs.

- Implementing incremental mkconsistent requires it's own separate state
  to detect mutation correctly since LFS_T_MKCONSISTENT and
  LFS_T_LOOKAHEAD are invalidated by slightly different things.

- If hasorphans=true, we're likely going to find orphans and clobber the
  traversal. So it's not really worth trying to opportunistically prove
  there are no orphans while doing other traversal operations.

- We don't really want to traverse the mroot/mtree during mkconsistent,
  which makes deduplicating these two functions a bit tricky. Doable,
  but annoying.

- grms don't involve traversals and are their own separate awkward step
  already.

Combine this with the fact that needing to scan for orphans should be
relatively rare in practice -- requiring either a powerloss or a
complicated set of file operations with at minimum 3 desynced files --
and parallel orphan checking starts to look like more trouble than it's
worth...

Instead, we now only check if the hasorphan bit has been set, and if it
has been we just call lfsr_fs_mkconsistent directly. This does a full
traversal in a single step, but at least makes it so traversal +
LFS_T_MKCONSISTENT in a background thread will do any necessary
janitorial work.

This saves a bit code:

           code          stack
  before: 35480           2680
  after:  35232 (-0.7%)   2680 (+0.0%)

lfs.h

@@ -179,8 +179,7 @@ enum lfs_traversal_flags {
 //    LFS_T_REPAIRDATA    = 0x0c00, // Repair metadata + data blocks
 
     // internally used flags
-    LFS_F_DIRTY         = 0x1000, // Filesystem modified during traversal
-    LFS_F_MUTATED       = 0x4000, // Filesystem modified by traversal
+    LFS_F_DIRTY         = 0x1000, // Filesystem has been modified
 };