Factor out common usage-help code

src/usage.cpp

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: MIT
+
+#include "usage.hpp"
+
+#include <stdio.h>
+#include <stdlib.h>
+
+void Usage::printAndExit(int code) const {
+	fputs(usage, stderr);
+	exit(code);
+}
+
+void Usage::printAndExit(char const *fmt, ...) const {
+	va_list args;
+	fputs("FATAL: ", stderr);
+	va_start(args, fmt);
+	vfprintf(stderr, fmt, args);
+	va_end(args);
+	putc('\n', stderr);
+
+	printAndExit(1);
+}