Add libString.

src/libString/s_append.c

@@ -0,0 +1,17 @@
+#include <u.h>
+#include <libc.h>
+#include "libString.h"
+
+/* append a char array to a String */
+String *
+s_append(String *to, char *from)
+{
+	if (to == 0)
+		to = s_new();
+	if (from == 0)
+		return to;
+	for(; *from; from++)
+		s_putc(to, *from);
+	s_terminate(to);
+	return to;
+}