analyze: add compare-versions

analyze: add compare-versions

The interface, output, and exit status convention are all taken directly from
rpmdev-vercmp and dpkg --compare-versions. The implementation is different
though. See test-string-util for a list of known cases where we compare
strings incompatibly.

The idea is that this string comparison function will be declared as "the"
method to use for boot entry ordering in the specification and similar
uses. Thus it's nice to allow users to compare strings.