While here, if we cannot find ypbind to work out how the hook script
should be configured, guess according to OS.

Even though we now have fully working kqueue(2) and epoll(7)
with our privsep code, ppoll(2) is faster and smaller for our
workload.

This time though, we will keep the code here as it's fully working
and while fixing kqueue we also fixed other bugs in dhcpcd as a result
so it's not time wasted at all.

eloop allows for O(1) processing of active fd's.
The problems with the prior implementation have now been fixed.

kqueue allows for O(1) processing of active fd's an a more
robust signal handling method without the need to use global
variables to avoid calling functions during signal delivery.

The problems with the prior implemenation have now been fixed.

Their ps tools go out of their way to only show how it was launched :/

Allows to force for example ypbind if it's not installed.