Add libString.

src/libString/s_putc.c

diff --git a/src/libString/s_putc.c b/src/libString/s_putc.c
new file mode 100644
index 0000000..29a385d
--- /dev/null
+++ b/src/libString/s_putc.c
@@ -0,0 +1,13 @@
+#include <u.h>
+#include <libc.h>
+#include "libString.h"
+
+void
+s_putc(String *s, int c)
+{
+	if(s->ref > 1)
+		sysfatal("can't s_putc a shared string");
+	if (s->ptr >= s->end)
+		s_grow(s, 2);
+	*(s->ptr)++ = c;
+}