iob_send sends the (rest of) b over the socket s.
iob_send returns the number of bytes written, 0 if there were no more
bytes to be written in the batch, -1 for EAGAIN, or -3 for a permanent
error (for example "connection reset by peer").
The normal usage pattern is using io_wait to know when a descriptor is
writable, and then calling iob_send until it returns 0, -1 or -3.
If it returns 0, terminate the loop (everything was written OK). If it
returns -1, call io_wait again. If it returned -3, signal an error and
close the socket.
The benefit of the I/O batch API is that it exploits platform specific
APIs like FreeBSDs sendfile. The file contents will always be sent in
a way that allows the operating systems to perform zero copy TCP, and
the buffers will always be sent using as few syscalls as possible and
avoiding unnecessary copying (using writev).