functions seemlessly work on fds that aren't for the main socket. This
involved changing some fd-variable names (to make them clearer), adding
io_set_sock_fds(), and making input buffering have a better enabled
flag (via an allocated buffer, just like the output buffering).  I also
got rid of the fd arg to some functions where the fd arg could only
specify the input or output fd for the socket (which we already know).