Cookies

I made a few small changes to the cookie support
(mainly one change and a couple of utily function added).

First of all, I added a new CookieMode: HT_COOKIE_SAME_DOMAIN. It works
like HT_COOKIE_SAME_HOST but only checks for the terminal part of the
hostname (possibly the domain name).

This allows to accept/send cookies to all the hosts in the same domain,
since many sites now split their content on multiple hosts.

Also I added a couple of new string utilities: tailcomp and tailcasecomp.
They act like strcmp and strcasecmp but check the first strings against
the end of the second string. This allows "domain comparison" as described
in the cookie specs.

The code is still missing all the checks for a proper domain (2 or 3 dots
depending on the top level), but it's ok for controlled use.

If you think this code can be useful I can check it in.

-- Raffaele