Fix mdb_txn_begin(&read-only txn) calls. Test mdb_env_set_maxreaders().
Rename DBI open/close functions. Move mdb_dbi_close() out of txn.

Just use srand/rand, we don't care about the quality
of the random numbers, we just want some data.

Initialize unused portions of DB pages, for security or to silence
checkers like Purify and Valgrind. Like BDB configure --enable-umrw.

When collapsing root, must also move cursor index down,
not just the page pointer.

Also in mtest, break from NEXT loops on error, otherwise it just
prints the previous key/data again, which looks confusing.

Make sure C_INITIALIZED gets set on successful call

Avoid naming conflicts with other mdb* packages. Bump version to 0.9.5

mdb_close() takes an env, not a txn.
Also, add check for stale DB records (in case some other process
wrote to the DB last).

Use sync writes in mtest.

Note derived from Martin Hedenfalk's btree.c; the code
is significantly reworked but owes a lot to the original.

NUMKEYS was size_t due to PAGEHDRSZ; printing unsigned int is simpler.

Instead increase PAGEFILL to long, since it involves a multiply.
Possibly uint32_t or something would be sufficient.