This code would previously use dns_entry->addr->ai_canonname
instead of the given host name, which caused us grief and
problems since not all our resolver options do the reverse lookup
and I would also guess that it caused problems with KRB5/GSS with
virtual name-based hosts. Now the host name from the URL is used.
Define HAVE_GSSMIT if <gssapi/{gssapi.h,gssapi_generic.h,gssapi_krb5.h}> are
available, otherwise define HAVE_GSSHEIMDAL if <gssapi.h> is available.
Only define GSS_C_NT_HOSTBASED_SERVICE to gss_nt_service_name if
GSS_C_NT_HOSTBASED_SERVICE isn't declared by the gssapi headers. This should
avoid breakage in case we wrongly recognize Heimdal as MIT again.
