UdpSocket.recv() using uninitialized Vec

[newfla]

Is your feature request related to a problem? Please describe.
As today UdpSocket.recv() and UdpSocket.recv_from() require a Vec<u8> with length > 0.
This prevents users to use a Vec<u8> created with Vec::with_capacity() for skipping the vec initialization; the methods always returns OK(0).

As workaround try_recv_buf_from and try_recv_buf are available but they increase the code complexity.

Describe the solution you'd like
I've considered the following alternatives:

1. Implements recv_buf and recv_buf_from: they will work with buf having length=0 without the complication of try variants.

Describe alternatives you've considered

2. Switching recv() and recv_from from &mut [u8] to &mut BufMut. I'm not sure if that's an acceptable solution

Additional context
As bonus point, it would be nice to improve methods documentations to underline the different behavior of the current methods: as today, the difference is only shown in the code snippets (not in the description).

[Darksonn]

I don't mind adding recv_buf and recv_buf_from.

Changing the definition of recv is definitely not an acceptable solution. That would be a breaking change.

[newfla]

I noticed that it is possible to add the same methods also to the UnixDatagram struct.
@Darksonn, If you don't see any problems, I'll prepare a pull request in the next few days

[Darksonn]

Thanks! Yes, they should be added to UnixDatagram too.