ITS#7793 mdb_cursor_put(): Fix MDB_CURRENT.

Ignore key, broken by 5bda3565a9bfaa6cd54053faeafcc06da15bc00c
and some older code.  Document and clarify MDB_CURRENT usage.

Also affects non-MDB_CURRENT put() with empty data and a key
which matches by the mdb_set_compare function but not by memcmp.
mtest[46]: Use different DBs when different flags.
mtest*.c, sample-*.c: Add/note error checks.
Merge remote-tracking branch 'mdb/mdb.master' into mdb.master