udp: add optional send/recv handler

[alfredh]

usecase: use udp_sock with external/custom sockets

[alfredh]

hi @sreimers

could you please take a look at this one

The usecase is to use external send handlers or external filedescriptors:

1. Application creates a socket (fd) and then allocates udp_sock with that FD.
2. Application creates udp_sock with custom send handler, and uses that one instead of FD.

Alfred

[sreimers]

Looks good, thanks!