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.

Make sure errors are propagated from init_meta

Loop on copyfd meta write, since pipes may return after partial write.

partial revert of d6d2638a and
26a25df5

The original code was already tested and working correctly.

Return the actual shared reader count when it exists, not
just the current process env's reader count.

NetBSD can only handle up to 14 chars, we were using 21. Now
we encode to 15, and for NetBSD truncate the last char.

Check again after acquiring rmutex. Avoids potential issue with
a duplicate pid coming in between initial check and rmutex.

Avoid holding rmutex for longer than necessary.

Use mti_numreaders for loop limit, not me_maxreaders.

Don't obtain reader txn before displaying reader table. Exit
after reader table if no other DB query options were given.

Dump the active slots in the reader table.

Retrieve the flags from a DB handle.

Don't count pages twice if they're already accounted in an ancestor txn.

Same fix for cursor_first/last.

Whenever we enter cursor_set() the sub-cursor's flag must be
cleared. If the new cursor position has valid subdata it will
be initialized again, if not then the sub-cursor has nothing
to point to.

This avoids unnecessary rewrites of pages that do not change.
(Restructuring for upcoming mdb_page_spill work.)

TODO: Rename C_UNTRACK to C_TRACKED.  Omitted now for readability.
The current name is because it's lazy: not always set when tracked.

(Restructuring for upcoming mdb_page_spill work.)
mdb_freelist_save() can't just Get() the destination, since
mdb_page_spill() may have put the destination in the read-only map.

TODO: Can this new put() modify the freelist, which would break it? The
final iteration's put() can shorten the node, the rest uses MDB_CURRENT.

We could set P_KEEP on dirty freeDB leaves and ovpages, since they are
all about to be modified.  But the code in this commit must stay anyway,
if mdb should support dropping a 256G DB. I.e. too big for dirty_list.

Commit d6d2638a broke read
on zero-length files.

Nor uninited cursors' subcursors' page pointers.

When collapsing root, must also move cursor index down,
not just the page pointer.

Also in mtest, break from NEXT loops on error, otherwise it just
prints the previous key/data again, which looks confusing.