Previously, we only looked at up to 128 bytes.  This is a bad idea
since socks messages can be at least 256+x bytes long.  Now we look at
up to 512 bytes; this should be enough for 0.2.2.x to handle all valid
SOCKS messages.  For 0.2.3.x, we can think about handling trickier
cases.

Fixes 2330.  Bugfix on 0.2.0.16-alpha.