Use gss_krb5_import_cred for remctl_set_ccache

If a Kerberos library and gss_krb5_import_cred are available at build
time, libremctl now uses them to implement remctl_set_ccache to avoid
affecting global program GSS-API state.  If those requirements are
met, remctl_set_ccache will only affect the remctl context on which
it's called.

This also requires importing the Kerberos portability layer, so make
sure that it's usable for the TAP Kerberos add-on.

Change-Id: I561812d0e36df6adf52d974dd5390953940865c5
Reviewed-on: https://gerrit.stanford.edu/1198
Reviewed-by: Russ Allbery <rra@stanford.edu>
Tested-by: Russ Allbery <rra@stanford.edu>