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.

Use fcntl() to set the flag.  Linux open(,O_DIRECT...) can create
the file even on failure, if the filesystem lacks O_DIRECT support.

Set rc=0 when taking the SET_RANGE branch

Must copy tmp cursor back to real cursor when merging into tmp cursor.

Reset delete flag in cursor_put().

If so, don't advance cursor on next NEXT. Also, init xcursor
on the next GET_CURRENT.

The caller is responsible for making sure no transactions are
active in this process before resizing. This is slightly lighter
weight than doing a full env_close/env_open cycle.

Use a txn instead of env argument. Only return persistent flags.