mdb_dcmp() = EINVAL?

[Hallvard Breien Furuseth <h.b.furuseth@usit.uio.no>]

mdb_dcmp() returns EINVAL if no dupsort function is set.
I'm not sure it does the caller any favors in doing so: It
claims (A>B && B>A) if it gets called with both (A,B) and
(B,A).  That can confuse the caller, e.g. a sort function.

I expect the sensible results without a dcmp function are
0 or a crash.  Plus to document this clearly.

If that EINVAL is useful, i.e. if there are times when the
user wishes to ask if a dupsort function is set, then mdb
can grow an mdb_dbi_info(env, dbi, &result) function to
describe the database: Name, flags, sort funcs.

-- 
Hallvard