Fix GSSAPI error handling (dotnet/corefx#34883)

Fix GSSAPI error handling (dotnet/corefx#34883)

This PR addresses 3 issues in GSSAPI error handling:

* Fixes an uninitialized variable when calling gss_display_status().
This was causing the major statuscode error message to always return
'An invalid status code was supplied'.

* If the minor statuscode is 0 (GSS_S_COMPLETE), we shouldn't be calling
for the minor statuscode error message text. That is why we were always getting
'Unknown error' for that text.

* Added more text (help text) to certain errors returned from GSSAPI
such as when the NTLM plugin is not installed.

No new tests were added to this PR since it requires machine changes
to test. However, I manually tested various combinations of errors
including when 'gss-ntlmssp' is installed or not.

In terms of dotnet/corefx#34877, the exception message goes from:

>GSSAPI operation failed with error - An invalid status code was supplied (Unknown error).

to

>GSSAPI operation failed with error - An unsupported mechanism was requested. NTLM authentication requires the GSSAPI plugin 'gss-ntlmssp'.

Fixes dotnet/corefx#34877

Commit migrated from https://github.com/dotnet/corefx/commit/eec001d96a68376c0e504eb7635c8edec196f90f