Netconn: when an encrypted connection is used, randomly set one or more of three...

Netconn: when an encrypted connection is used, randomly set one or more of three unused NETFLAGs.

When AES encryption is enabled, this will then work around substring
matching in routers/IPS thanks to CBC and our method of IV selection.
See "startkeylogger". Note that this neither improves nor weakens
security - SSL e.g. has the very issue we're fixing here because it is
based on TCP, and thus resends always are identical payloads.

git-svn-id: svn://svn.icculus.org/twilight/trunk/darkplaces@12023 d7cf8633-e32d-0410-b094-e92efae38249