changelog and old vulnerability

Hey,

News on the web site as of today:

I made a single changelog page and concatenated all release-notes since 1.4.0
into that so that we have a single page that is easier to search:

  https://c-ares.haxx.se/changelog.html

For the sake of better visibility, I also created a new page to more
distingtly list known security problems in previous versions:

  https://c-ares.haxx.se/vulns.html

-- 
  / daniel.haxx.se