[krbdev.mit.edu #8770] Constify more of the GSS-API

[Nico Williams via RT]

GSS functions pre-RFC5587 can also benefit from using gss_const_* types.
Heimdal has changed many of them to do so.

The input_cred_handle argument to gss_add_cred(), however, should not be
gss_const_cred_id_t.

This change is source and binary backwards-compatible for applications,
but it's not source backwards-compatible for mechanisms.