Disallow aliasing of return value and modulus

Disallow aliasing of return value and modulus

All the functions changed in this commit would silently misbehave if the
return value aliases the modulus, most of the time they would succeed and
return an incorrect result of 0 in that situation. This adjusts all the
functions in BN_mod.c, others and documentation will follow later.

Prompted by a bug report about BN_mod_inverse() by Guido Vranken.

ok jsing