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.

xP/Makefile

@@ -0,0 +1,14 @@
+.POSIX:
+.SUFFIXES:
+
+outputs = xP proto.go public/mithril.js
+all: $(outputs)
+
+xP: xP.go proto.go
+	go build -o $@
+proto.go: ../xC-gen-proto.awk ../xC-gen-proto-go.awk ../xC-proto
+	awk -f ../xC-gen-proto.awk -f ../xC-gen-proto-go.awk ../xC-proto > $@
+public/mithril.js:
+	curl -Lo $@ https://unpkg.com/mithril/mithril.js
+clean:
+	rm -f $(outputs)