Test for broken counter-mode at runtime

To solve bug 4779, we want to avoid OpenSSL 1.0.0's counter mode.
But Fedora (and maybe others) lie about the actual OpenSSL version,
so we can't trust the header to tell us if it's safe.

Instead, let's do a run-time test to see whether it's safe, and if
not, use our built-in version.

fermenthor contributed a pretty essential fixup to this patch. Thanks!