http_negotiate_sspi: Use a dynamic buffer for SPN generation

Updated to use a dynamic buffer for the SPN generation via the recently
introduced Curl_sasl_build_spn() function rather than a fixed buffer of
1024 characters, which should have been more than enough, but by using
the new function removes the need for another variable sname to do the
wide character conversion in Unicode builds.
http_negotiate_sspi: Tidy up to remove the get_gss_name() function

Due to the reduction of code in commit 3b924b29 of get_gss_name() the
function isn't necessary anymore.