[FEATURE] Polling support for Unix datagram sockets

### Is your feature request related to a problem? Please describe.

This is a followup of [!16555](https://github.com/apache/nuttx/issues/16555) that solved the issue of non working DGRAM sockets.

### Describe the solution you'd like

Polling for Unix datagram sockets (`SOCK_DGRAM`) is currently not implemented. The implementation to `local_netpoll.c` seems pretty straightforward (at least for `POLLIN`), just create receiver if it doesn't exist (default state for the datagram, basically the same as in `psock_dgram_recvfrom` and call `file_poll`. The behavior of poll is still not as expected though, because it returns `POLLHUP` right after the poll is called. But this event should not return on datagram sockets as they are not connection oriented.

This bring us back to the same issue @Cynerd already faced with stream sockets in https://github.com/apache/nuttx/pull/14667. NuttX sockets are implemented with fifos and pipes, but the expected behavior of local sockets (both stream and dgram) is a bit different from pipes.

```diff
diff --git a/net/local/local_netpoll.c b/net/local/local_netpoll.c
index e68833aa9a..264c59c28a 100644
--- a/net/local/local_netpoll.c
+++ b/net/local/local_netpoll.c
@@ -161,7 +161,29 @@ int local_pollsetup(FAR struct socket *psock, FAR struct pollfd *fds)

if (conn->lc_proto == SOCK_DGRAM)
{
- return -ENOSYS;
+ if (conn->lc_infile.f_inode == NULL)
+ {
+ ret = local_create_halfduplex(conn, conn->lc_path, conn->lc_rcvsize);
+ if (ret < 0)
+ {
+ nerr("ERROR: Failed to create FIFO for %s: %d\n",
+ conn->lc_path, ret);
+ return ret;
+ }
+
+ /* Open the receiving side of the transfer */
+
+ ret = local_open_receiver(conn, false);
+ if (ret < 0)
+ {
+ nerr("ERROR: Failed to open FIFO for %s: %d\n",
+ conn->lc_path, ret);
+ local_release_halfduplex(conn);
+ return ret;
+ }
+ }
+
+ return file_poll(&conn->lc_infile, fds, true);
}

#ifdef CONFIG_NET_LOCAL_STREAM
@@ -310,7 +332,7 @@ int local_pollteardown(FAR struct socket *psock, FAR struct pollfd *fds)

if (conn->lc_proto == SOCK_DGRAM)
{
- return -ENOSYS;
+ return file_poll(&conn->lc_infile, fds, false);
}

#ifdef CONFIG_NET_LOCAL_STREAM
```

The implementation of polling for DGRAM would probably require the rewrite of local sockets and reimplementation with internet socket code base instead of fifos/pipes.

### Describe alternatives you've considered

_No response_

### Verification

- [x] I have verified before submitting the report.