Update the way we handle http_host keywords.

Previously we would have forced all users to use nocase with http_host
keywords(since the hostname buffer is lowercase).

We now error out on sigs that has nocase set with http_host set.  Also if
the http_host pattern or http_host pcre has an uppercase character set, we
invalidate such sigs.  Unittests also updated to reflect the above change.
Minor SigValidate cleanup