gpg: Allow for longer signature subpackets.

* g10/parse-packet.c (parse_signature): Increase the cap for hashed
subpackets to 30000.  Print the value in the error message.  Do not
return an error but skip a too long signature.

The limit of 10000 served us well for decades but given the option to
put a key into the signature, a larger limit will eventually be
useful. The second part makes things a bit robust against rogue
subpackets on a keyserver.