crypto: avoid generating control-like packets

crypto: avoid generating control-like packets

In non-AES mode, we now replace packets whose hash starts with FFFFFFFF by
7FFFFFFF, and 8000LLLL by 0000LLLL. When decoding, packets with these prefixes
have two attempts.

AES encrypted mode is unaffected by this, as there packets always start with a
byte from 00 to 0F.

Also, no longer whine about receiving packets that fail the HMAC test when the
packets look like control packets (the NQ CONNECT packet DP sends when
connecting sometimes was received in time for this to match).

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@11068 d7cf8633-e32d-0410-b094-e92efae38249