Add ability to drop overly large packets

As per draft-dkg-openpgp-abuse-resistant-keystore add the ability to
drop "large" packets. These are defined as UIDs more than 1024 bytes
long, UATs greater than 64k in size and all other packets larger than
8383 bytes (the maximum that will fit in a 2 byte new format packet
length). Disabled by default, enable with "check_packet_size" in the
verification config section.