(ITS#7598) Named MDB database incompatible with some flags

[h.b.furuseth@usit.uio.no]

Full_Name: Hallvard B Furuseth
Version: mdb.master, a2ce25482a60e516d1d9b417ed666dac6fdebb93
OS: Linux x86_64
URL: 
Submission from: (NULL) (193.69.163.163)
Submitted by: hallvard


mdb_dbi_open(named database) and thus "mdb_stat -a"
crash if the main database has the MDB_DUPSORT flag:
It hits assert(mx != NULL) in mdb_cursor_init().

Fixing to reject mdb_dbi_open(named database) in this
case, and also with MDB_INTEGERKEY which would expect
the char* database name to be a binary integer.