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/c/mfcalc/Makefile

@@ -0,0 +1,25 @@
+# This Makefile is designed to be simple and readable.  It does not
+# aim at portability.  It requires GNU Make.
+
+BASE = mfcalc
+BISON = bison
+XSLTPROC = xsltproc
+
+all: $(BASE)
+
+%.c %.xml %.gv: %.y
+	$(BISON) $(BISONFLAGS) --xml --graph=$*.gv -o $*.c $<
+
+%: %.c
+	$(CC) $(CFLAGS) -o $@ $<
+
+run: $(BASE)
+	@echo "Type arithmetic expressions.  Quit with ctrl-d."
+	./$<
+
+html: $(BASE).html
+%.html: %.xml
+	$(XSLTPROC) $(XSLTPROCFLAGS) -o $@ $$($(BISON) --print-datadir)/xslt/xml2xhtml.xsl $<
+
+clean:
+	rm -f $(BASE) $(BASE).c $(BASE).html $(BASE).xml $(BASE).gv $(BASE).output