bmap: Initial scaffolding for on-disk block map

This is pretty exploratory work, so I'm going to try to be less thorough
in commit messages until the dust settles.

---

New tag for gbmapdelta:

  LFS3_TAG_GBMAPDELTA   0x0104  v--- ---1 ---- -1rr

New tags for in-bmap block types:

  LFS3_TAG_BMRANGE      0x033u  v--- --11 --11 uuuu
  LFS3_TAG_BMFREE       0x0330  v--- --11 --11 ----
  LFS3_TAG_BMINFLIGHT   0x0331  v--- --11 --11 ---1
  LFS3_TAG_BMINUSE      0x0332  v--- --11 --11 --1-
  LFS3_TAG_BMBAD        0x0333  v--- --11 --11 --11
  LFS3_TAG_BMERASED     0x0334  v--- --11 --11 -1--

New gstate decoding for gbmap:

  .---+- -+- -+- -+- -. cursor: 1 leb128  <=5 bytes
  | cursor            | known:  1 leb128  <=5 bytes
  +---+- -+- -+- -+- -+ block:  1 leb128  <=5 bytes
  | known             | trunk:  1 leb128  <=4 bytes
  +---+- -+- -+- -+- -+ cksum:  1 le32    4 bytes
  | block             | total:            23 bytes
  +---+- -+- -+- -+- -'
  | trunk         |
  +---+- -+- -+- -+
  |     cksum     |
  '---+---+---+---'

New bmap node revdbg string:

  vvv---- -111111- -11---1- -11---1-  (62 62 7e v0  bb~r)  bmap node

New mount/format/info flags (still unsure about these):

  LFS3_M_BMAPMODE     0x03000000  On-disk block map mode
  LFS3_M_BMAPNONE     0x00000000  Don't use the bmap
  LFS3_M_BMAPCACHE    0x01000000  Use the bmap to cache lookahead scans
  LFS3_M_BMAPSLOW     0x02000000  Use the slow bmap algorithm
  LFS3_M_BMAPFAST     0x03000000  Use the fast bmap algorithm

New gbmap wcompat flag:

  LFS3_WCOMPAT_GBMAP  0x00002000  Global block-map in use

lfs3_util.h

@@ -47,6 +47,9 @@
 #ifndef LFS3_GC
 #define LFS3_GC
 #endif
+#ifndef LFS3_BMAP
+#define LFS3_BMAP
+#endif
 #endif
 
 // LFS3_YES_* variants imply the relevant LFS3_* macro
@@ -80,6 +83,26 @@
 #ifdef LFS3_YES_GC
 #define LFS3_GC
 #endif
+#ifdef LFS3_YES_BMAP
+#define LFS3_BMAP
+#endif
+
+// TODO is this the best way to structure this?
+// LFS3_BMAP mappings
+//
+// LFS3_YES_BMAP => LFS3_YES_BMAPFAST
+#ifdef LFS3_YES_BMAP
+#ifndef LFS3_YES_BMAPFAST
+#define LFS3_YES_BMAPFAST
+#endif
+#endif
+// LFS3_YES_BMAP* => LFS3_BMAP
+#if defined(LFS3_YES_BMAPFAST) \
+        || defined(LFS3_YES_BMAPSLOW) \
+        || defined(LFS3_YES_BMAPCACHE) \
+        || defined(LFS3_YES_BMAPNONE)
+#define LFS3_BMAP
+#endif
 
 // LFS3_NO_LOG disables all logging macros
 #ifdef LFS3_NO_LOG
@@ -257,6 +280,12 @@
 #define LFS3_IFDEF_GC(a, b) (b)
 #endif
 
+#ifdef LFS3_BMAP
+#define LFS3_IFDEF_BMAP(a, b) (a)
+#else
+#define LFS3_IFDEF_BMAP(a, b) (b)
+#endif
+
 
 // Some function attributes, no way around these