Fix #2261 by supporting Red Hat's limited PATH

Red Hat has this theory that binaries in sbin are too dangerous to be on
the default path, but we need them anyway.

RH7 has /sbin and /usr/sbin as symlinks, so that is no longer important.

I'm adding it to the `PATH` instead of making the path to `modinfo`
absolute because I am worried about breaking support for other
distributions.