Reduced the scope of LFS3_REVDBG/REVNOISE

LFS3_REVDBG introduced a lot of overhead for something I'm not sure
anyone will actually use (I have enough tooling that the state of an
rbyd is rarely a mystery, see dbgbmap.py). That, and we're running out
of flags!

So this reduces LFS3_REVDBG to just store one of "himb" in the first
(lowest) byte of the revision count; information that is easily
available:

  vvvv---- -------- -------- --------
  vvvvrrrr rrrrrr-- -------- --------
  vvvvrrrr rrrrrrnn nnnnnnnn nnnnnnnn
  vvvvrrrr rrrrrrnn nnnnnnnn dddddddd
  '-.''----.----''----.- - - '---.--'
    '------|----------|----------|---- 4-bit relocation revision
           '----------|----------|---- recycle-bits recycle counter
                      '----------|---- pseudorandom noise (if revnoise)
                                 '---- h, i, m, or b (if revdbg)
                             -11-1---  - h = mroot anchor
                             -11-1--1  - i = mroot
                             -11-11-1  - m = mdir
                             -11---1-  - b = btree node

Some other notes:

- Enabled LFS3_REVDBG and LFS3_REVNOISE to work together, now that
  LFS3_REVDBG doesn't consume all unused rev bits.

  Note that LFS3_REVDBG has priority over LFS3_REVNOISE, but _not_
  recycle-bits, etc. Otherwise problems would happen for recycle-bits
  >2^20 (though do we care?).

- Fixed an issue where using the gcksum as a noise source results in
  noise=0 when there is only an mroot. This is due to how we xor out
  the current mdir cksum during an mdir commit.

  Fixed by using gcksum_p instead of gcksum.

- Added missing LFS3_I_REVDBG/REVNOISE flags in the tests, so now you
  can actually run the tests with LFS3_REVDBG/REVNOISE (this probably
  just fell out-of-date at some point).

---

Curiously, despite LFS3_REVDBG/REVNOISE being disabled by default, this
did save some code. I'm guessing the non-tail-call mtree/gbmap commit
functions prevented some level of inlining?:

                 code          stack          ctx
  before:       35964           2280          660
  after:        35964 (+0.0%)   2280 (+0.0%)  660 (+0.0%)

                 code          stack          ctx
  gbmap before: 38940           2296          772
  gbmap after:  38828 (-0.3%)   2296 (+0.0%)  772 (+0.0%)

tests/test_gc.toml

@@ -1885,7 +1885,9 @@ code = '''
     struct lfs3_fsinfo fsinfo;
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            LFS3_I_MKCONSISTENT
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | LFS3_I_MKCONSISTENT
                 | LFS3_I_RELOOKAHEAD
                 | ((GBMAP)
                     ? (LFS3_IFDEF_GBMAP(LFS3_I_REGBMAP, -1)
@@ -2028,7 +2030,9 @@ code = '''
     // did these clear the right flags?
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
                 | ((!RELOOKAHEAD) ? LFS3_I_RELOOKAHEAD : 0)
                 | ((!REGBMAP)
                     ? ((GBMAP)
@@ -2116,7 +2120,9 @@ code = '''
     struct lfs3_fsinfo fsinfo;
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            LFS3_I_MKCONSISTENT
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | LFS3_I_MKCONSISTENT
                 | LFS3_I_RELOOKAHEAD
                 | ((GBMAP)
                     ? (LFS3_IFDEF_GBMAP(LFS3_I_REGBMAP, -1)
@@ -2259,7 +2265,9 @@ code = '''
     // did these clear the right flags?
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
                 | ((!RELOOKAHEAD) ? LFS3_I_RELOOKAHEAD : 0)
                 | ((!REGBMAP)
                     ? ((GBMAP)
@@ -2279,7 +2287,9 @@ code = '''
     // check that flags were reset
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            LFS3_I_MKCONSISTENT
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | LFS3_I_MKCONSISTENT
                 | LFS3_I_RELOOKAHEAD
                 | ((REGBMAP)
                     ? LFS3_IFDEF_GBMAP(LFS3_I_REGBMAP, -1U)
@@ -2426,7 +2436,9 @@ code = '''
     // did these clear the right flags?
     lfs3_fs_stat(&lfs3, &fsinfo) => 0;
     assert(fsinfo.flags == (
-            ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
+            LFS3_IFDEF_YES_REVDBG(LFS3_I_REVDBG, 0)
+                | LFS3_IFDEF_YES_REVNOISE(LFS3_I_REVNOISE, 0)
+                | ((!MKCONSISTENT) ? LFS3_I_MKCONSISTENT : 0)
                 | ((!RELOOKAHEAD) ? LFS3_I_RELOOKAHEAD : 0)
                 | ((!REGBMAP)
                     ? ((GBMAP)