Changed all dir tests to be reentrant

To help with this, added TEST_PL, which is set to true when powerloss
testing. This way tests can check for stronger conditions (no EEXIST)
when not powerloss testing.

With TEST_PL, there's really no reason every test in t5_dirs shouldn't
be reentrant, and this gives us a huge improvement of test coverage very
cheaply.

---

The increased test coverage caught a bug, which is that gstate wasn't
being consumed properly when mtree uninlining. Humorously, this went
unnoticed because the most common form of mtree uninlining, mdir splitting,
ended up incorrectly consuming the gstate twice, which canceled itself
out since the consume operation is basically just xor.

Also added support for printing dstarts to dbglfs.py, to help debugging.

scripts/dbgmtree.py

@@ -488,7 +488,8 @@ class Rbyd:
         return tree, t_depth
 
     # btree lookup with this rbyd as the root
-    def btree_lookup(self, f, block_size, bid, depth=None):
+    def btree_lookup(self, f, block_size, bid, *,
+            depth=None):
         rbyd = self
         rid = bid
         depth_ = 1
@@ -543,7 +544,8 @@ class Rbyd:
                 return not tags, bid + (rid_-rid), w, rbyd, rid_, tags, path
 
     # btree rbyd-tree generation for debugging
-    def btree_tree(self, f, block_size, depth=None, *,
+    def btree_tree(self, f, block_size, *,
+            depth=None,
             inner=False):
         # find the max depth of each layer to nicely align trees
         bdepths = {}
@@ -671,7 +673,8 @@ class Rbyd:
         return tree, max((branch.d+1 for branch in tree), default=0)
 
     # btree B-tree generation for debugging
-    def btree_btree(self, f, block_size, depth=None, *,
+    def btree_btree(self, f, block_size, *,
+            depth=None,
             inner=False):
         # find all branches
         tree = set()