Changed CKSUM suptype encoding from 0x2000 -> 0x3000

I may be overthinking things, but I'm guessing of all the possible tag
modes we may want to add in the future, we will mostly like want to add
something that looks vaguely tag like. Like the shrub tags, for example.

It's beneficial, ordering wise, for these hypothetical future tags to
come before the cksum tags.

Current tag modes:

  0x0ttt  v--- tttt -ttt tttt  normal tags
  0x1ttt  v--1 tttt -ttt tttt  shrub tags
  0x3tpp  v-11 tttt ---- ---p  cksum tags
  0x4kkk  v1dc kkkk -kkk kkkk  alt tags

scripts/dbgmtree.py

@@ -42,8 +42,8 @@ TAG_DID         = 0x032c
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0600
 TAG_SHRUB       = 0x1000
-TAG_CKSUM       = 0x2000
-TAG_ECKSUM      = 0x2100
+TAG_CKSUM       = 0x3000
+TAG_ECKSUM      = 0x3100
 TAG_ALT         = 0x4000
 TAG_GT          = 0x2000
 TAG_R           = 0x1000
@@ -335,7 +335,7 @@ class Rbyd:
                     weight = weight_
 
             # evaluate trunks
-            if (tag & 0xe000) != TAG_CKSUM and (
+            if (tag & 0xf000) != TAG_CKSUM and (
                     not trunk or trunk >= j_-d or wastrunk):
                 # new trunk?
                 if not wastrunk: