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.