add iob_write (to write an io batch through a callback)

test/iob.c

@@ -1,17 +1,24 @@
 #include <assert.h>
 #include "iob.h"
 #include "buffer.h"
+#include <unistd.h>
+
+static int64 write_cb(int64 fd,const void* buf,uint64 len) {
+  if (len>2) len=2;
+  return write(fd,buf,len);
+}
 
 int main() {
   int64 fd;
   io_batch* b;
+  int64 r;
   assert(io_readfile(&fd,"GNUmakefile"));
   assert(b=iob_new(10));
   assert(iob_addbuf(b,"Huhu",4));
   assert(iob_addbuf(b," fnord\n",7));
   assert(iob_addfile(b,fd,10,10));
-  buffer_puts(buffer_2,"iob_send returned ");
-  buffer_putlong(buffer_2,iob_send(1,b));
-  buffer_putsflush(buffer_2,".\n");
+  do {
+    r=iob_write(1,b,write_cb);
+  } while (r>0);
   return 0;
 }