examples: sort them per language and complete them

Convert some of the READMEs to Markdown, which is now more common, and
nicely displayed in some git hosting services.

Add missing READMEs and Makefiles.  Generate XML, HTML and Dot files.  Be
sure to ship the test files.  Complete CLEANFILES to remove all generated
files.

* examples/calc++: Move into...
* examples/c++: here.
* examples/mfcalc, examples/rpcalc: Move into...
* examples/c: here.

* examples/README.md, examples/c++/calc++/Makefile, examples/c/local.mk,
* examples/c/mfcalc/Makefile, examples/c/rpcalc/Makefile,
* examples/d/README.md, examples/java/README.md:
New files.

* examples/test (medir): Be robust to deeper directory nesting.

examples/d/README.md

@@ -0,0 +1,28 @@
+# Examples in D
+
+This directory contains examples of Bison grammar files in D.
+
+You can run `make` to compile these examples.  And `make clean` to tidy
+afterwards.
+
+## d/calc.y
+The usual calculator.
+
+<!---
+
+Local Variables:
+fill-column: 76
+ispell-dictionary: "american"
+End:
+
+Copyright (C) 2018 Free Software Foundation, Inc.
+
+Permission is granted to copy, distribute and/or modify this document
+under the terms of the GNU Free Documentation License, Version 1.3 or
+any later version published by the Free Software Foundation; with no
+Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
+Texts.  A copy of the license is included in the "GNU Free
+Documentation License" file as part of this distribution.
+
+# LocalWords:  mfcalc calc parsers yy
+--->