This extends 2d79a0bbb9 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.
0004f698df