gpg: Limit keysize for unattended key generation to useful values.

* g10/keygen.c (gen_elg): Enforce keysize 1024 to 4096.
(gen_rsa): Enforce keysize 1024 to 4096.
(gen_dsa): Enforce keysize 768 to 3072.

It was possible to create 16k RSA keys in batch mode. In addition to the
silliness of such keys, they have the major drawback that under GnuPG
and Libgcrypt, with their limited amount of specially secured memory
areas, the use of such keys may lead to an "out of secure memory"
condition.