udev: do not pass timeout_warn_usec around, calculate it on demand

udev: do not pass timeout_warn_usec around, calculate it on demand

It was always set to one third of timeout_usec, so let's simplify things by
calculating it using a helper function right before it is used.

Before 9d9264ba39f797d20100c8acfda3df895ab5aaa2, udevd.c would avoid setting
timeout_warn_usec to 0, using 1 instead. This wasn't necessary, because when
timeout_warn_usec is finally used in spawn_wait(), it is ignored if
timeout_usec is 0 or timeout_warn_usec is 0. So there was no need to handle
this case specially.