Client capabilities are specified using the "*cap" info key.  The info string
is made up of single char flags (possibly with modifiers, though currently
none exist).

Defined capabilities (* = not implemented):
z   client can accept gzipped files.
h   http transfers
f   * ftp transfers
a   * audio channel (voice chat)
i   * irc
p   pogo stick control
t   team messages

For more information on z and h, see qw-http-spec.txt.

The QuakeForge clients will not send "*cap" to the server unless "QF" or "EXT"
is detected in the challenge string sent by the server.

From the QuakeForge quakeworld server:

    if (sv_extensions->int_val) {
        extended = " QF qtv EXT";
    }

    // send it to the client
    Netchan_OutOfBandPrint (net_from, "%c%i%s", S2C_CHALLENGE,
                            svs.challenges[i].challenge, extended);