gssapi: import bugfixes from Apple Heimdal-520

* check `ctx->gc_ctx` in `gss_inquire_context()`
* check `gm_inquire_cred != NULL` in `gss_inquire_context()`
* check `min_lifetime` in `gss_inquire_cred()`
* check `gm_inquire_cred_by_mech != NULL` in `gss_inquire_cred_by_mech()`
* set mech error in `gss_inquire_cred_by_oid()`
* don't clobber error in `gss_inquire_cred_by_oid()`
* don't pass NULL minor_status to `gss_krb5_free_lucid_sec_context()`
* allow NULL ccache in `gss_krb5_ccache_name()`
* NULL names OK in `_gss_find_mn()`
* allow empty names in `gss_import_name()` (removes `input_name_buffer` length
  check). to support ANONYMOUS.  in `gss_import_name()`, ignore mech name
  import failure as long as it's possible to import the name in some other
  mechanism
* better argument validation in `gss_export_sec_context()`
* in `gss_compare_name()`, check `mn2 != NULL`
* check `gss_add_oid_set_member()` return code in `gss_indicate_mechs()`
* in `gss_destroy_cred()`, set output cred handle to `GSS_C_NO_CREDENTIAL`
* cast size_t to OM_uint32 where required

lib/gssapi/mech/gss_cred.c

@@ -158,7 +158,7 @@ gss_import_cred(OM_uint32 * minor_status,
 	    goto out;
 	}
 	oid.elements = data.data;
-	oid.length = data.length;
+	oid.length = (OM_uint32)data.length;
 
 	m = __gss_get_mechanism(&oid);
 	krb5_data_free(&data);