Get the BN_mod_exp_mont bugfix (for handling negative inputs) correct
this time.

Fix BN_gcd.
Analyze BN_mod_inverse.
Add BN_kronecker.
"make update".

two functions that did expansion on in parameters (BN_mul() and
BN_sqr()).  The problem was solved by making bn_dup_expand() which is
a mix of bn_expand2() and BN_dup().

temporary BIGNUMs. BN_CTX still uses a fixed number of BIGNUMs, but
the BN_CTX implementation could now easily be changed.