Variables: fixed non-indexed access of prefix vars (ticket #600).

Previously, a configuration like

    location / {
        ssi on;
        ssi_types *;
        set $http_foo "bar";
        return 200 '<!--#echo var="http_foo" -->\n';
    }

resulted in NULL pointer dereference in ngx_http_get_variable() as
the variable was explicitly added to the variables hash, but its
get_handler wasn't properly set in the hash.  Fix is to make sure
that get_handler is properly set by ngx_http_variables_init_vars().
Variables: updated list of prefixes in ngx_http_rewrite_set().