Fix DSA parameter generation to use the correct loop bound.
Fix DSA parameter generation to use the correct loop bound.

4096 is the value from FIPS 186-2, FIPS 186-3 uses 4*pbits which
is the obvious extension of the FIPS 186-2 scheme to larger parameter
sizes. Pointed out by @neverhub

Removes support for 512 and 768 bit DSA groups because WTF no.

webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,randombit/botan