#9 add mcrypt_set_sbox function

[vertur]

There is not defined substitution unit table for gost-28147-89 in standarts. So, some peoples use one tables, but other use others. It would be very nice to setup own s-table for gost-28147-89 algorithm.

I attached patch, that add new api function mcrypt_set_sbox. This function is allow setup custom sbox table for gost algorithm. This patch must be applied after gost_fixed.patch (see BUG #41).