Fixed ENOSPC issues with zero-granularity blocks

Result of testing on zero-granularity blocks, where the prog size and
read size equals the block size. This represents SD cards and other
traditional forms of block storage where we don't really get a benefit
from the metadata logging.

Unfortunately, since updates in both are tested by the same script,
we can't really use simple bash commands. Added a more complex
script to simulate corruption. Fortunately this should be more robust
than the previous solutions.

The main fixes were around corner cases where the commit logic fell
apart when it didn't have room to complete commits, but these were
fixable in the current design.

tests/corrupt.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python
+
+import struct
+import sys
+import os
+
+def main(*paths):
+    # find most recent block
+    file = None
+    rev = None
+    for path in paths:
+        try:
+            nfile = open(path, 'r+b')
+            nrev, = struct.unpack('<I', nfile.read(4))
+
+            assert rev != nrev
+            if not file or ((rev - nrev) & 0x80000000):
+                file = nfile
+                rev = nrev
+        except IOError:
+            pass
+
+    # go to last commit
+    tag = 0
+    while True:
+        try:
+            ntag, = struct.unpack('<I', file.read(4))
+        except struct.error:
+            break
+
+        tag ^= ntag
+        file.seek(tag & 0xfff, os.SEEK_CUR)
+
+    # lob off last 3 bytes
+    file.seek(-((tag & 0xfff) + 3), os.SEEK_CUR)
+    file.truncate()
+
+if __name__ == "__main__":
+    main(*sys.argv[1:])