Support new $Girocco::Config::disable_dsa setting

Support new $Girocco::Config::disable_dsa setting

OpenSSH DSA keys are problematic in that ever since OpenSSH version
4.3p1 only 1024 bit DSA keys can be created with ssh-keygen.

Even if a longer DSA key is created (with an older OpenSSH version or
with OpenSSL), the SSH protocol requires use of the SHA-1 hash in the
DSA signature blob so the value of longer DSA keys is of somewhat
questionable value when used with the SSH protocol.

If the new $Girocco::Config::disable_dsa setting is true then both
reguser.cgi and edituser.cgi will prevent DSA keys from being stored.

Pre-existing DSA keys will continue to work regardless of the current
$Girocco::Config::disable_dsa setting since the keys are only prohibited
at reguser.cgi/edituser.cgi time.