[RFE] Add a way to application to know a minimum DH size allowed by the client

[Tomas Mraz]

The current minimum DH size allowed by the client is 768 bits which is a
hardcoded constant. It would be nice if the constant was at least
#define in public headers or even better if there was an API to query
various minimum and maximum bit sizes that are checked in the library
such as the maximum supported key lengths, etc.

-- 
Tomas Mraz
No matter how far down the wrong road you've gone, turn back.
                                              Turkish proverb
(You'll never know whether the road is wrong though.)