sasl_sspi: Fixed memory leak with not releasing Package Info struct

Curl_sasl_create_digest_md5_message() wouldn't free the Package Info
structure after QuerySecurityPackageInfo() had allocated it.
sasl: Introduced Curl_sasl_build_spn() for building a SPN

Various parts of the libcurl source code build a SPN for inclusion in
authentication data. This information is either used by our own native
generation routines or passed to authentication functions in third-party
libraries such as SSPI. However, some of these instances use fixed
buffers rather than dynamically allocated ones and not all of those that
should, convert to wide character strings in Unicode builds.

Implemented a common function that generates a SPN and performs the
wide character conversion where necessary.
sasl_sspi: Fixed SPN not being converted to wchar under Unicode builds

Curl_sasl_create_digest_md5_message() would simply cast the SPN variable
to a TCHAR when calling InitializeSecurityContext(). This meant that,
under Unicode builds, it would not be valid wide character string.

Updated to use the recently introduced Curl_sasl_build_spn() function
which performs the correct conversion for us.
sasl: Use a dynamic buffer for SPN generation

Updated Curl_sasl_create_digest_md5_message() to use a dynamic buffer
for the SPN generation via the recently introduced Curl_sasl_build_spn()
function rather than a fixed buffer of 128 characters.
sasl: Tidy up to rename SPN variable from URI
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.