Parse "timeout=0" as infinity in various generators (#6264)

This extends 2d79a0bb to the kernel
command line parsing.

The parsing is changed a bit to only understand "0" as infinity. If units are
specified, parse normally, e.g. "0s" is just 0. This makes it possible to
provide a zero timeout if necessary.

Simple test is added.

Fixes https://bugzilla.redhat.com/show_bug.cgi?id=1462378.