Various C files: Ignore write errors of UDP and IP datagrams.

Various C files: Ignore write errors of UDP and IP datagrams.

These packets are expected to go missing periodically and everyone will
cope.  Unfortunately, GCC wants us to do something with the return code
from write(2), so we explicitly assign it to a write-only variable and
hope that its data-flow analysis is done after it checks for return-code
ignoring.