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  just fell out-of-date at some point).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%)

lfs3_util.h

@@ -215,12 +215,24 @@
 #define LFS3_IFDEF_REVDBG(a, b) (b)
 #endif
 
+#ifdef LFS3_YES_REVDBG
+#define LFS3_IFDEF_YES_REVDBG(a, b) (a)
+#else
+#define LFS3_IFDEF_YES_REVDBG(a, b) (b)
+#endif
+
 #ifdef LFS3_REVNOISE
 #define LFS3_IFDEF_REVNOISE(a, b) (a)
 #else
 #define LFS3_IFDEF_REVNOISE(a, b) (b)
 #endif
 
+#ifdef LFS3_YES_REVNOISE
+#define LFS3_IFDEF_YES_REVNOISE(a, b) (a)
+#else
+#define LFS3_IFDEF_YES_REVNOISE(a, b) (b)
+#endif
+
 #ifdef LFS3_CKPROGS
 #define LFS3_IFDEF_CKPROGS(a, b) (a)
 #else