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.

xF.svg

@@ -0,0 +1,36 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg version="1.1" width="48" height="48" viewBox="0 0 48 48"
+   xmlns:xlink="http://www.w3.org/1999/xlink"
+   xmlns="http://www.w3.org/2000/svg"
+   xmlns:svg="http://www.w3.org/2000/svg">
+
+  <defs>
+    <linearGradient id="background" x1="0" y1="0" x2="1" y2="1">
+      <stop stop-color="#808080" offset="0" />
+      <stop stop-color="#000000" offset="1" />
+    </linearGradient>
+    <!-- librsvg screws up the filter's orientation in a weird way
+         otherwise a larger blur value would look better -->
+    <filter id="shadow" color-interpolation-filters="sRGB">
+      <feOffset dy="0.5" />
+      <feGaussianBlur stdDeviation="0.5" />
+      <feComposite in2="SourceGraphic" operator="in" />
+    </filter>
+    <clipPath id="clip">
+      <rect x="-7" y="-10" width="14" height="20" />
+    </clipPath>
+  </defs>
+
+  <circle cx="24" cy="24" r="20"
+     fill="url(#background)" stroke="#404040" stroke-width="2" />
+
+  <g transform="rotate(-45 24 24)" filter="url(#shadow)">
+    <path d="m 12,25 h 24 v 11 h -5 v -8 h -4.5 v 6 h -5 v -6 h -9.5 z"
+       fill="#ffffff" />
+    <g stroke-width="4" transform="translate(24, 16)" clip-path="url(#clip)"
+       stroke="#ffffff">
+      <line x1="-8" x2="8" y1="-5" y2="5" />
+      <line x1="-8" x2="8" y1="5" y2="-5" />
+    </g>
+  </g>
+</svg>