hub/connection.c: Use a smaller read buffer

There's really no need to have a 8k read buffer for hub connections.
Only when logging in will the hub send a significant amount of data (the
user list, MOTD), but that doesn't justify having a large read buffer
for the remaining part of the connection. Even on large hubs the regular
traffic rarely exceeds 1kb/s, so a 1kb buffer suffices.
netstream.c: Fix two zlibr-related bugs; hub/adc.c: Add ZLIF support