Added optional per-header length limits:

Added optional per-header length limits:

 - libwebsockets.h:
 - * added struct lws_token_limits
   * added token limits pointer to lws_context_creation_info
 - private-libwebsockets.h: added token limits pointer to lws_context_creation_info
 - context.c: copy token limits in create_context
 - client.c / server.c: pass context when invoking libwebsocket_parse
 - parsers.c:
 - * libwebsocket_parse takes context pointer
   * issue_char takes context pointer and checks header length against context limits, if defined
   * issue_char returns 1 (not -1/0) for header too long, and spill: sets the state to WSI_TOKEN_SKIPPING