The variable was considered non-existent in the absence of any
valid_referers directives.

Given the following config snippet,

    location / {
        return 200 $invalid_referer;
    }

    location /referer {
        valid_referers server_names;
    }

"location /" should work identically and independently on other
"location /referer".

The fix is to always add the $invalid_referer variable as long
as the module is compiled in, as is done by other modules.

Because of premature check the effective buffer size was 255 symbols
while the buffer is able to handle 256.

This allows to approach "server_name" values specified below the
"valid_referers" directive when used within the "server_names" parameter, e.g.:

    server_name  example.org;
    valid_referers server_names;
    server_name  example.com;

As a bonus, this fixes bogus error with "server_names" specified several times.

The server_name regexes are normally compiled for case-sensitive matching.
This violates case-insensitive obligations in the referer module.  To fix
this, the host string is converted to lower case before matching.

Previously server_name regex was executed against the whole referer string
after dropping the scheme part.  This could led to an improper matching, e.g.:

    server_name ~^localhost$;
    valid_referers server_names;

    Referer: http://localhost/index.html

It was changed to look only at the hostname part.

The server_name regexes are separated into another array to not clash with
regular regexes.

When matching a compiled regex against value in the "Referer" header field,
the length was calculated incorrectly for strings that start from "https://".
This might cause matching to fail for regexes with end-of-line anchors.

Patch by Liangbin Li.

This makes it accessible via dynamic lookup with ngx_http_get_variable()
from Perl, SSI, etc.

Patch by Yichun Zhang (agentzh).