Skip cursors that aren't as deep as current cursor

Tweak split_indx check, go one slot further.

In d8eccb35

Return the environment's filedescriptor. Useful when the caller
is doing their own locking.

USE_POSIX_SEM must define USE_HASH

Also, check the split point on branch pages as well as leaf pages.

There may be other uses for it besides semaphore names.

Other cursors being fixed up are not necessarily height 1.

Don't allow shrinking below minimum size for an already open env.

Do not fail after mdb_page_new() succeeds.

Both were unused and md_name was unmaintained -- except
mdb_cursor_touch(xcursor) would abuse md_name as a key to
touch MAIN_DBI if it could somehow get passed ! DB_DIRTY.

xcursor DBIs were parent DBI+1 for debugging. Instead output
-(parent DBI).  Fixes a crash in mdb_cursor_del0()'s xcursor
tracking, it forgot to subtract 1 for C_SUB cursors.

Leave all lock management to the caller.

Calling app wants to manage its own locking.

MDB_txn.mt_toggle: Use (mt_txnid & 1) instead.
Drop error checks which will be repeated.
mdb_cursor_set(): Turn assert into if/return to match the above.
mdb_cursor_del(): 'flags' are now used as bitflags.

Only named DBs can have DB_STALE, and they do not use MDB_PS_MODIFY.
Replace magic key values with flags.  Drop duplicated comments at
mdb_page_search_root() vs. mdb_page_search(), and rephrase.

Happened since sqlightning #includes mdb.c, midl.c.

The names have caused bugs, "txn" was treated as
the current transaction.

Malloc a page in this txn, not in a parent.

Catch malloc error.
Fix hunt for dirty vs spilled pages: Don't leave x at a deleted pageno.
Cleanup: Factor out variables, squash pages already marked for deletion.

Fix description & code: Also ignore dirty pages hidden by
spilled pages, as they won't merge into our dirty_list.
Update it in mdb_page_flush() instead of mdb_page_spill().

Don't prod sub-pages or pages referring to uninitialized xcursors.