introduce LWS_CALLBACK_CLIENT_APPEND_HANDSHAKE_HEADER

introduce LWS_CALLBACK_CLIENT_APPEND_HANDSHAKE_HEADER

This adds a callback to protocols[0] which happens when the
Client HTTP handshake packet is being composed.  After all the
headers for the websocket handshake to the server have been
added, the callback is called with a pointer to a char *
that allows extra headers to be added.  See the comments in
libwebsocket.h or the api documentation for example code.

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