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.

If mdb_page_touch() sees a page in txn's dirty_list, that
is the page version txn's cursors should have.  Fail if
the user may be seeing and depending on another version.

Restore mc_flags and xcursors, they were tracked but not merged.
Simplify: Track parent txn's original cursors after backing them
up, instead of tracking copies and merging them back at commit.

Terminate loop on intermediate failures, return count of written items,
document usage.

Page leak, mdb_page_alloc(). On error, don't shorten me_pghead.
Memleak, mdb_ovpage_free(). Free page or keep it in dirty_list.
Bad MIDL, mdb_midl_need(). Fix midl[-1] (allocated size).

Catch I/O errors. Do nothing between OS call failure and ErrCode().
Do not use errno after non-OS-errors like write() >= 0, which could
give a failure return of success (errno 0) or some irrelevant error
code.  Drop seek calls, use pwrite/pread/Windows OVERLAPPED offset.

Don't put a 64-bit filesize in a 32-bit int before shifting
down.  Always pass &sizehi to SetFilePointer->maxsize, so
sizelo not is treated a signed distance.  Hide unused vars
when _WIN32.  Reinitialize OVERLAPPED before reuse.

Grow midls earlier in order to catch errors earlier.  Use
mdb_midl_need() instead of mdb_midl_grow(), then mdb_midl_xappend()
needs no error checks. Factor out mdb_midl_append_range().