Implemented infrastructure necessary for global-removes

This has, in theory, global-removes (grm) being written out as a part of
of directory creation, but they aren't used in any form and so may not
be being written correctly.

But it did require quite a bit of problem solving to get to this point
(the interactions between mtree splitsand grms is really annoying), so
it's worth a commit.

scripts/dbgrbyd.py

@@ -20,6 +20,8 @@ COLORS = [
 TAG_NULL        = 0x0000
 TAG_SUPERMAGIC  = 0x0003
 TAG_SUPERCONFIG = 0x0004
+TAG_GSTATE      = 0x0100
+TAG_GRM         = 0x0100
 TAG_NAME        = 0x0200
 TAG_BRANCH      = 0x0200
 TAG_DSTART      = 0x0201
@@ -130,6 +132,12 @@ def tagrepr(tag, w, size, off=None):
         return 'superconfig%s %d' % (
             ' w%d' % w if w else '',
             size)
+    elif (tag & 0xff00) == TAG_GSTATE:
+        return '%s%s %d' % (
+            'grm' if tag == TAG_GRM
+                else 'gstate 0x%02x' % (tag & 0xff),
+            ' w%d' % w if w else '',
+            size)
     elif (tag & 0xff00) == TAG_NAME:
         return '%s%s %d' % (
             'branch' if tag == TAG_BRANCH