Allow generation of card keys up to 4096 bit.

This patch implementes a chunk mode to pass the key parameters from
scdaemon to gpg. This allows to pass arbitrary long key paremeters;
it is used for keys larger than 3072 bit.

Note: the card key generation in gpg is currently broken. The keys
are generated but it is not possible to create the self-signature
because at that time the gpg-agent does not yet know about the new
keys and thus can't divert the sign request to the card. We either
need to run the learn command right after calling agent_scd_genkey or
implement a way to sign using the currently inserted card. Another
option would be to get rid of agent_scd_genkey and implement the
feature directly in agent_genkey.