Raise OverflowError for invalid integers (match CFFI).
Introduce py_bool() helper.
Implement Environment.flags().
Undo braindamage from last commit.
First bits of env_test.
Fix CFF exception class map.

None of the relevant error constants were defined, and the previous loop
to build the map intentionally ignored errors. This fixes all that up.
Sort various lists of constants.
d8eccb353 @ git://git.openldap.org/openldap.git

ITS#7733 check cursor index before cursor_del
ITS#7734 Add MDB_NORDAHEAD to CHANGELESS.
Re-enable fixed test.
Raise ReadonlyError when MDB_RDONLY and begin(write=True)

Previously generic lmdb.Error was raised.
Don't mkdir() if Environment(readonly=True)
Fix cffi __del__ crash when txn constructor fails

Exception AttributeError: "'Transaction' object has no attribute '_txn'"
in <bound method Transaction.__del__ of <lmdb.cffi.Transaction object at
0x10961afd0>> ignored
Fix Environment(mode=) handling.

Previously mode= was passed straight to LMDB and never used during
create=True's mkdir. Now instead mode specifies the dir permissions,
which has -x masked off before passing to LMDB for use as the file mask.
Add Environment.max_key_size().
Add Environment.max_readers().
Don't build error_map if reading_docs().
Correct max_readers() doc.
Environment constructor test
Ensure consistent umask during mode tests.
Octal syntax compat fixes.