Drop UNLOCK_MUTEX_R() on mdb_reader_pid() error
ITS#7515 has been fixed.
ITS#7665 Drop lmdb.h doc on non-overlapping flags.

It was inaccurate and misplaced. Verify it in mdb.c instead.
ITS#7772 Reduce spurious sub-page growth.

mdb_cursor_put(): Use free sub-page space added by a
previous put(existing DUPSORT item), if any.
Tweak NUMKEYS code to avoid a new signedness warning.
Add mdb_dbg_pgno(): (sub)pagenum for debug output.
sprintf format: pid_t need not be an int.
ITS#7771 mdb_cursor_del0(): Fix xcursor tracking.