option: add function to format a list of short and long options together with their respective descriptions

Makefile

@@ -14,6 +14,7 @@ obj-y += source/file.o
 obj-y += source/image.o
 obj-y += source/info.o
 obj-y += source/main.o
+obj-y += source/option.o
 obj-y += source/pack.o
 obj-y += source/sign.o
 obj-y += source/unpack.o

include/option.h

@@ -0,0 +1,9 @@
+#pragma once
+
+struct opt_desc {
+	const char *short_name;
+	const char *long_name;
+	const char *desc;
+};
+
+void format_options(struct opt_desc *entries);

source/option.c

@@ -0,0 +1,15 @@
+#include <stdio.h>
+
+#include <option.h>
+
+void format_options(struct opt_desc *entries)
+{
+	struct opt_desc *entry;
+
+	for (entry = entries; entry->long_name; ++entry) {
+		fprintf(stderr, "%7s %-16s %s\n",
+			entry->short_name ? entry->short_name : "",
+			entry->long_name,
+			entry->desc);
+	}
+}