Caret comparison

[behrmann]

Looking at the reference implementation in systemd-analyze compare-versions the code handling the caret character is the same as for - and ., right before and after:

        /* Handle '-', which separates version and release, e.g 123.4-3.1.fc33.x86_64 */
        if (*a == '-' || *b == '-') {
                /* The string prefixed with '-' is older (e.g., 123-9 vs 123.1-1) */
                r = CMP(*a != '-', *b != '-');
                if (r != 0)
                        return r;

                a++;
                b++;
        }

        /* Handle '^'. Used for patched release. */
        if (*a == '^' || *b == '^') {
                r = CMP(*a != '^', *b != '^');
                if (r != 0)
                        return r;

                a++;
                b++;
        }

        /* Handle '.'. Used for point releases. */
        if (*a == '.' || *b == '.') {
                r = CMP(*a != '.', *b != '.');
                if (r != 0)
                        return r;

                a++;
                b++;
        }

so the caret character must be treated the same in the spec as well. This is not a bug in the reference implementation, since when the sign of the comparison is inverted, i.e. caret compares higher than not-caret, the comparison

• 123^post1 < 123.a-1

would not hold true anymore, since ^ would win against . at step 5. This comparison must hold true, to be compatible with rpmdev-vercmp

Note: rpmdev-vercmp does compare 123^post1 < 123.a-1, but compares ^ > ., which systemd-analyze compares ^ < .

This PR also adds an extended example taken from a comment of the function implementing this in systemd-analyze, which I found helpful.