recv revert treating zero as hangup

While checking with ab, I found
commit 30cdb3ac
Author: Justin Chen <justinchen00@github.invalid.com>
Date:   Thu Apr 14 21:40:53 2016 +0800

    recv treat zero return as error

    https://github.com/warmcat/libwebsockets/issues/475



turned ab performance to crap, reverting it made everything fast again.

recv manpage says there is three ways to get zero returned

1)       When a stream socket peer has performed an orderly shutdown, the return value will be 0  (the  traditional  "end-of-file"
       return).

2)       Datagram  sockets  in  various  domains  (e.g., the UNIX and Internet domains) permit zero-length datagrams.  When such a
       datagram is received, the return value is 0.

3)       The value 0 may also be returned if the requested number of bytes to receive from a stream socket was 0.

we can't just assume it means the peer shut down.

If the peer shut down, then the event loop should get an event on the socket like POLLHUP and deal with it that way.

So the patch mentioned above is simply reverted here.

Signed-off-by: Andy Green <andy@warmcat.com>