Add libString.

src/libString/s_reset.c

@@ -0,0 +1,23 @@
+#include <u.h>
+#include <libc.h>
+#include "libString.h"
+
+String*
+s_reset(String *s)
+{
+	if(s != nil){
+		s = s_unique(s);
+		s->ptr = s->base;
+		*s->ptr = '\0';
+	} else
+		s = s_new();
+	return s;
+}
+
+String*
+s_restart(String *s)
+{
+	s = s_unique(s);
+	s->ptr = s->base;
+	return s;
+}