Don't clear readable flag on poll

When the slave becomes readable we set the pollin flag, and cleared it
in ttynvt_poll().
So if ttynvt_poll() is called twice, the readable flag was lost.

Now we clear it in ttynvt_read() if there actually are no more data to
be read.

ttynvt.c

@@ -597,7 +597,7 @@ ttynvt_read(fuse_req_t req, size_t size, off_t off,
 {
     ttynvt_t       *tty = (ttynvt_t *) (uintptr_t) info->fh;
     char            buf[TMP_BUF_SIZE];
-    int             res;
+    int             res, nr;
 
     DBG2("%s\n", __func__);
 
@@ -626,6 +626,16 @@ ttynvt_read(fuse_req_t req, size_t size, off_t off,
     tty->ptid_read = 0;
     tty->n_fds = 3;             /* Enable polling of Slave */
     fuse_req_interrupt_func(req, NULL, NULL);
+    if (res < (int)size)
+    {
+        tty->pollin = 0;
+    }
+    else
+    {
+        nr = 0;
+        ioctl(tty->fds[FD_SLAVE].fd, FIONREAD, &nr);
+        tty->pollin = nr > 0;
+    }
     if (res < 0)
     {
         DBG2("%s: error: %m\n", __func__);
@@ -1057,7 +1067,6 @@ ttynvt_poll(fuse_req_t req, struct fuse_file_info *info,
 
     if (tty->pollin)
     {
-        tty->pollin = 0;
         revents |= POLLIN;
     }
     if (tty->error)