Add support for fopenf(), formatted string fopen()

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>

tests/fopenf.c

@@ -0,0 +1,18 @@
+#include "check.h"
+
+int main(void)
+{
+	FILE *fp;
+	char *dir = "/tmp";
+	char *file = "__foo__";
+
+	fp = fopenf("w", "%s/%s", dir, file);
+	if (fp) {
+		fputs("foo", fp);
+		fclose(fp);
+	}
+
+	fail_unless(fexist("/tmp/__foo__"));
+
+	return 0;
+}