net: Add recv_buf for UdpSocket and UnixDatagram

[newfla]

Motivation

Closes #5563.

In order to supplies buffer with length=0 to receive data on UdpSocket and UnixDatagram, try_recv_buf_from and try_recv_buf should be used.
Unfortunately, they require additional care compared to standard recv variants.

Solution

Two new methods recv_buf and recv_buf_from have been added to UdpSocket and UnixDatagram.
These provide the same ergonomic of recv and recv_from, without the constraint buf.length > 0.

[Darksonn]

I have a few comments. Note that the issues below appear multiple times.

[Darksonn]

Thanks, LGTM!

[Darksonn]

There's a CI failure:

$ rustfmt --check --edition 2018 $(git ls-files '*.rs'); then
Diff in /home/runner/work/tokio/tokio/tokio/src/net/udp.rs at line 982:
                     unsafe { &mut *(dst as *mut _ as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };
 
                 let n = (*self.io).recv(dst)?;
-                
+
                 // Safety: We trust `UdpSocket::recv` to have filled up `n` bytes in the
                 // buffer.
                 unsafe {
Please run `rustfmt --edition 2018 $(git ls-files '*.rs')` to fix rustfmt errors.
See CONTRIBUTING.md for more details.

[newfla]

There's a CI failure:

$ rustfmt --check --edition 2018 $(git ls-files '*.rs'); then
Diff in /home/runner/work/tokio/tokio/tokio/src/net/udp.rs at line 982:
                     unsafe { &mut *(dst as *mut _ as *mut [std::mem::MaybeUninit<u8>] as *mut [u8]) };
 
                 let n = (*self.io).recv(dst)?;
-                
+
                 // Safety: We trust `UdpSocket::recv` to have filled up `n` bytes in the
                 // buffer.
                 unsafe {
Please run `rustfmt --edition 2018 $(git ls-files '*.rs')` to fix rustfmt errors.
See CONTRIBUTING.md for more details.

I accidentally inserted whitespaces in the previous commit. Fixed :)

[Darksonn]

Thanks!