For consistency with the spnego and oauth2 code moved the setting of
the host name outside of the Curl_auth_create_gssapi_user_messag()
function.
This will allow us to more easily override it in the future.
Also display the GSS_C_GSS_CODE (major code) when specified instead of
only GSS_C_MECH_CODE (minor code).
In addition, the old code was printing a colon twice after the prefix
and also miscalculated the length of the buffer in between calls to
gss_display_status (the length of ": " was missing).
Also, gss_buffer is not guaranteed to be NULL terminated and thus need
to restrict reading by its length.
Closes#738
