gbmap: Tried to adopt the gbmap name more consistently

Having gbmap/bmap used in different places for the same thing was
confusing. Preferring gbmap as it is consistent with other gstate (grm
queue, gcksums), even if it is a bit noisy.

It's interesting to note what didn't change:

- The BM* range tags: LFS3_TAG_BMFREE, etc. These already differs from
  the GBMAP* prefix enough, and adopting GBM* would risk confusion for
  actual gstate.

- The gbmap revdbg string: "bb~r". We don't have enough characters for
  anything else!

- dbgbmap.py/dbgbmapsvg.py. These aren't actually related to the gbmap,
  so the name difference is a good thing.

tests/test_btree.toml

@@ -2,7 +2,7 @@
 after = 'test_rbyd'
 
 # don't bother testing with more complicated block allocators
-ifndef = 'LFS3_BMAP'
+ifndef = 'LFS3_GBMAP'
 
 # maximize lookahead buffer, we don't actually gc so we only get one pass
 # of the disk for these tests