Start X11 and web frontends for xC

For this, we needed a wire protocol.  After surveying available options,
it was decided to implement an XDR-like protocol code generator
in portable AWK.  It now has two backends, per each of:

 - xF, the X11 frontend, is in C, and is meant to be the primary
   user interface in the future.

 - xP, the web frontend, relies on a protocol proxy written in Go,
   and is meant for use on-the-go (no pun intended).

They are very much work-in-progress proofs of concept right now,
and the relay protocol is certain to change.

xC-proto

@@ -0,0 +1,120 @@
+// Backwards-compatible protocol version.
+const VERSION = 1;
+
+// From the frontend to the relay.
+struct CommandMessage {
+	u32 command_seq;
+	union CommandData switch (enum Command {
+		HELLO,
+		PING,
+		ACTIVE,
+		BUFFER_COMPLETE,
+		BUFFER_INPUT,
+		BUFFER_ACTIVATE,
+		BUFFER_LOG,
+	} command) {
+	case HELLO:
+		u32 version;
+	case PING:
+		void;
+	case ACTIVE:
+		void;
+	case BUFFER_COMPLETE:
+		string buffer_name;
+		string text;
+		u32 position;
+	case BUFFER_INPUT:
+		string buffer_name;
+		string text;
+	case BUFFER_ACTIVATE:
+		string buffer_name;
+	case BUFFER_LOG:
+		string buffer_name;
+	} data;
+};
+
+// From the relay to the frontend.
+struct EventMessage {
+	u32 event_seq;
+	union EventData switch (enum Event {
+		PING,
+		BUFFER_UPDATE,
+		BUFFER_RENAME,
+		BUFFER_REMOVE,
+		BUFFER_ACTIVATE,
+		BUFFER_LINE,
+		BUFFER_CLEAR,
+		ERROR,
+		RESPONSE,
+	} event) {
+	case PING:
+		void;
+	case BUFFER_UPDATE:
+		string buffer_name;
+	case BUFFER_RENAME:
+		string buffer_name;
+		string new;
+	case BUFFER_REMOVE:
+		string buffer_name;
+	case BUFFER_ACTIVATE:
+		string buffer_name;
+	case BUFFER_LINE:
+		string buffer_name;
+		bool is_unimportant;
+		bool is_highlight;
+		enum Rendition {
+			BARE,
+			INDENT,
+			STATUS,
+			ERROR,
+			JOIN,
+			PART,
+		} rendition;
+		// Unix timestamp in seconds.
+		u64 when;
+		// Broken-up text, with in-band formatting.
+		union ItemData switch (enum Item {
+			TEXT,
+			RESET,
+			FG_COLOR,
+			BG_COLOR,
+			FLIP_BOLD,
+			FLIP_ITALIC,
+			FLIP_UNDERLINE,
+			FLIP_INVERSE,
+			FLIP_CROSSED_OUT,
+			FLIP_MONOSPACE,
+		} kind) {
+		case TEXT:
+			string text;
+		case RESET:
+			void;
+		case FG_COLOR:
+			i16 color;
+		case BG_COLOR:
+			i16 color;
+		case FLIP_BOLD:
+		case FLIP_ITALIC:
+		case FLIP_UNDERLINE:
+		case FLIP_INVERSE:
+		case FLIP_CROSSED_OUT:
+		case FLIP_MONOSPACE:
+			void;
+		} items<>;
+	case BUFFER_CLEAR:
+		string buffer_name;
+	case ERROR:
+		u32 command_seq;
+		string error;
+	case RESPONSE:
+		u32 command_seq;
+		union ResponseData switch (Command command) {
+		case BUFFER_COMPLETE:
+			u32 start;
+			string completions<>;
+		case BUFFER_LOG:
+			// UTF-8, but not guaranteed.
+			u8 log<>;
+		} data;
+	} data;
+};