add buffer_fromsa (make virtual buffer from stralloc)

buffer/buffer_fromsa.c

@@ -0,0 +1,15 @@
+#include "stralloc.h"
+#include "buffer.h"
+
+static int dummyreadwrite(int fd,char* buf,unsigned long int len) {
+  return 0;
+}
+
+void buffer_fromsa(buffer* b,stralloc* sa) {
+  b->x=sa->s;
+  b->p=0;
+  b->n=sa->len;
+  b->a=sa->a;
+  b->fd=0;
+  b->op=dummyreadwrite;
+}