rfc822: Add an example program

Add a simple full-blown example program for the rfc822 module.
'headernames' simply dumps the header field name for every header field in
each message file given on the command line to stdout.
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

ccan/rfc822/examples/.gitignore

+headernames

ccan/rfc822/examples/Makefile

+CFLAGS= -I../../.. ../../rfc822.o \
+	../../talloc/talloc.o ../../grab_file/grab_file.o \
+	../../list/list.o ../../noerr/noerr.o
+
+EXAMPLES = headernames
+
+all: $(EXAMPLES)
+
+clean:
+	rm -f $(EXAMPLES)

ccan/rfc822/examples/headernames.c

+#include <stdio.h>
+#include <stdlib.h>
+#include <stdarg.h>
+#include <string.h>
+
+#include <errno.h>
+#include <unistd.h>
+
+#include <ccan/talloc/talloc.h>
+#include <ccan/grab_file/grab_file.h>
+#include <ccan/rfc822/rfc822.h>
+
+static void process_file(const char *name)
+{
+	void *ctx = talloc_new(NULL);
+	size_t size;
+	void *buf;
+	struct rfc822_msg *msg;
+	struct rfc822_header *hdr;
+
+	buf = grab_file(ctx, name, &size);
+
+	msg = rfc822_start(ctx, buf, size);
+
+	rfc822_for_each_header(msg, hdr) {
+		struct bytestring hname = rfc822_header_raw_name(msg, hdr);
+
+		printf("%.*s\n", hname.len, hname.ptr);
+	}
+
+	talloc_free(ctx);
+}
+
+int main(int argc, char *argv[])
+{
+	int i;
+
+	for (i = 0; i < (argc - 1); i++)
+		process_file(argv[i + 1]);
+
+	exit(0);
+}