Added several more LFS_DEBUG* options

To help with debugging. These all seem useful, though the exact output
will probably be worth messing around with:

- LFS_DEBUGRBYDFETCHES - Debug every rbyd fetch
- LFS_DEBUGRBYDCOMMITS - Debug every rbyd commit
- LFS_DEBUGBTREEFETCHES - Debug every btree/bshrub fetch (though we
  currently don't fetch bshrubs...)
- LFS_DEBUGBTREECOMMITS - Debug every btree/bshrub commit
- LFS_DEBUGMDIRFETCHES - Debug every mdir fetch
- LFS_DEBUGMDIRCOMMITS - Debug every mdir commit
- LFS_DEBUGALLOCS - Debug every block allocation

Let's see if you can match these to each debug output:

  lfs.c:2942:debug: Fetched rbyd 0xe.d80 w77, eoff 3536, cksum 862283c6
  lfs.c:4233:debug: Committed rbyd 0xe.dd0 w78, eoff 3616, cksum 38ae1347
  lfs.c:4950:debug: Fetched btree 0x9f.806 w2048, cksum 7fb89b1b
  lfs.c:6609:debug: Committed btree 0x9f.806 w2048, cksum 7fb89b1b
  lfs.c:6603:debug: Committed bshrub 0x{0,1}.b06 w1747
  lfs.c:7290:debug: Fetched mdir -1 0x{1,0}.8f w0, cksum 7846be7a
  lfs.c:9022:debug: Committed mdir 0 0x{0,1}.a10 w2, cksum 4d2ccb29
  lfs.c:10083:debug: Allocated block 0x8f, lookahead 125/253/256

Also tweaked LFSR_DEBUGRBYDBALANCE to be a bit more readable when
LFS_DEBUGRBYDFETCHES is enabled, and tweaked the out-of-space error
message to show the same lookahead info as LFS_DEBUGALLOCS:

  lfs.c:10101:error: No more free space (lookahead 0/0/256)
                                                   ^ ^  ^
                             lookahead remaining --' |  |
                             ckpoint remaining ------'  |
                             block count ---------------'

No code changes.

lfs_util.h

@@ -204,12 +204,6 @@ extern "C"
 #define LFS_IFDEF_GC(a, b) (b)
 #endif
 
-#ifdef LFS_DEBUGRBYDBALANCE
-#define LFS_IFDEF_DEBUGRBYDBALANCE(a, b) (a)
-#else
-#define LFS_IFDEF_DEBUGRBYDBALANCE(a, b) (b)
-#endif
-
 
 // Builtin functions, these may be replaced by more efficient
 // toolchain-specific implementations. LFS_NO_BUILTINS falls back to a more