Renamed file_cache -> fcache and gbmap_re -> regbmap

This walks back some of the attempt at strict object namespacing in
struct lfs3_cfg:

- cfg.file_cache_size  -> cfg.fcache_size
- filecfg.cache_size   -> filecfg.fcache_size
- filecfg.cache_buffer -> filecfg.fcache_buffer
- cfg.gbmap_re_thresh  -> cfg.regbmap_thresh

Motivation:

- cfg.regbmap_thresh now matches cfg.gc_regbmap_thresh, instead of using
  awkwardly different namespacing patterns.

- Giving fcache a more unique name is useful for discussion. Having
  pcache, rcache, and then file_cache was a bit awkward.

  Hopefully it's also more clear that cfg.fcache_size and
  filecfg.fcache_size are related.

- Config in struct lfs3_cfg is named a bit more consistently, well, if
  you ignore gc_*_* options.

- Less typing.

Though this gets into pretty subjective naming territory. May revert
this if the new terms are uncomfortable after use.

tests/test_mount.toml

@@ -274,8 +274,8 @@ defines.RELOOKAHEAD = [false, true]
 defines.CKMETA = [false, true]
 defines.CKDATA = [false, true]
 defines.SIZE = [
-    'FILE_CACHE_SIZE/2',
-    '2*FILE_CACHE_SIZE',
+    'FCACHE_SIZE/2',
+    '2*FCACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -365,7 +365,7 @@ defines.RELOOKAHEAD = [false, true]
 defines.COMPACTMETA = [false, true]
 defines.CKMETA = [false, true]
 defines.CKDATA = [false, true]
-defines.SIZE = 'FILE_CACHE_SIZE/2'
+defines.SIZE = 'FCACHE_SIZE/2'
 # <=2 => grm-able
 #  >2 => requires orphans
 defines.ORPHANS = [0, 1, 2, 3, 100]
@@ -480,8 +480,8 @@ code = '''
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'FILE_CACHE_SIZE/2',
-    '2*FILE_CACHE_SIZE',
+    'FCACHE_SIZE/2',
+    '2*FCACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',
@@ -569,8 +569,8 @@ done:;
 defines.N = [1, 2, 4, 8, 16, 32, 64]
 defines.SIZE = [
     '0',
-    'FILE_CACHE_SIZE/2',
-    '2*FILE_CACHE_SIZE',
+    'FCACHE_SIZE/2',
+    '2*FCACHE_SIZE',
     'BLOCK_SIZE/2',
     'BLOCK_SIZE',
     '2*BLOCK_SIZE',