ws protocol name matching: fix Sec-WebSocket-Protocol: reply,  make a zero-length protocol name match any requested protocol


Also includes #157: 

if an empty string as protocol name in struct libwebsocket_protocols is
specified, it will match any protocol the client requested.

A later FILTER_PROTOCOL_CONNECTION callback can then be used to sort out details, like
setting options in the user session structure based on the actual proto name.

The previous two commits are minor - error message improvements.