[krbdev.mit.edu #7985] Add krb5_get_init_creds_opt_set_pac_request

[Greg Hudson via RT]

Heimdal has a function krb5_get_init_creds_opt_set_pac_request which 
causes a PA-PAC-REQUEST padata to be included in the AS request, 
requesting the inclusion or exclusion of a PAC.  We should support this 
as well.

See also:

http://mailman.mit.edu.ezproxyberklee.flo.org/pipermail/krbdev/2014-August/012138.html