libpq: Be strict about cancel key lengths

libpq: Be strict about cancel key lengths

The protocol documentation states that the maximum length of a cancel
key is 256 bytes. This starts checking for that limit in libpq.
Otherwise third party backend implementations will probably start
using more bytes anyway. We also start requiring that a protocol 3.0
connection does not send a longer cancel key, to make sure that
servers don't start breaking old 3.0-only clients by accident. Finally
this also restricts the minimum key length to 4 bytes (both in the
protocol spec and in the libpq implementation).

Author: Jelte Fennema-Nio <postgres@jeltef.nl>
Reviewed-by: Jacob Champion <jchampion@postgresql.org>
Discussion: https://www.postgresql.org/message-id/df892f9f-5923-4046-9d6f-8c48d8980b50@iki.fi
Backpatch-through: 18