Don't compare cursors beyond their depth. (detected by valgrind)

Should allow for the possibility of future flag combos.

If a key has a single existing value, and then a put (MDB_MULTIPLE)
is done where the first of the multiple values matches the existing
value, the put would return SUCCESS without writing any of the
values. Fixed to loop to the next value as intended.

Use double DPRINTF() parenthesis, so varargs macros are not needed.
Parenthesize MDB_DEBUG, in case the user defines it as foo || bar.

Also check xcursor instead of DUPSORT, it's slightly briefer.

Checking dirty_list was insufficient after a spill
with no named databases and no positioned cursors.

Howard Chu authored Aug 09, 2013 and Quanah Gibson-Mount committed Aug 09, 2013

countryString and telephoneNumber were missing

Return EINVAL only for simple programmer errors.

Ignore parent txn cursors since it is the current txn's dirty_list
which will be flushed. But check the current txn also when clearing,
since cursors can have pages which are dirty in a parent.

Check !mc_xcursor instead of !MDB_DUPSORT.  Equivalent for valid
data, but a bit safer if the sub-DB flags are corrupt.

Ensure me_pghead has room before removing from spill/dirty list.
Don't return pages to me_pghead in nested txns, use mt_free_pgs.

mdb_page_spill(): Don't binary-search the unsorted dirty_list.
mdb_page_flush(): Don't overwrite unprocessed dirty_list items.

Pid locking needs a different lockfile-version: MDB_env's with and
without pid locking must not coexist, they can sabotage each other.
Store MDB_LOCK_FORMAT = (version | "use locking" flag) instead.

Treat unexpected errors as "don't know".  Invert Pidcheck return
value, so nonzero including error codes = "the process may exist".

On Windows: Catch exited but still existing processes.  Handle
undefined PROCESS_QUERY_LIMITED_INFORMATION.
On Unix: don't trust F_GETLK error to leave the input alone,
the fcntl() doc seems unclear.

mdb_copy: Does not copy lockfile. Can trigger file growth.
mdb_txn_begin(): Clarify usage restrictions.
mdb_drop(): State what to do rather than what will be done, since
closing the handle could otherwise be read as happening even at failure.