Add a file for shared functions

New common.[hc] files where shared functions between surf and
webkitextension will be put. First addition is die().

common.c

@@ -0,0 +1,15 @@
+#include <stdarg.h>
+#include <stdio.h>
+#include <stdlib.h>
+
+void
+die(const char *errstr, ...)
+{
+	va_list ap;
+
+	va_start(ap, errstr);
+	vfprintf(stderr, errstr, ap);
+	va_end(ap);
+	exit(1);
+}
+