Instead of having duplicate functions that are type custom use a signle common
function that also performs access to the cache as the user owner, implicitly
validating correctness of ownership.

Resolves:
https://fedorahosted.org/sssd/ticket/2061

This completely replaces the per-ccache-type custom code to remove old cacches
and instead uses libkrb5 base doperations (krb5_cc_destroy) and operating as
the user owner.

Resolves:
https://fedorahosted.org/sssd/ticket/2061

This function safely destroy a ccache given a cache name and user crdentials.
It becomes the user so no possible races can compromise the system, then
uses libkrb5 functions to properly destroy a ccache, independently of the
cache type.
Finally restores the original credentials after closing the ccache handlers.

Resolves:
https://fedorahosted.org/sssd/ticket/2061

In some cases we want to temporarily assume user credentials but allow the
process to regain back the original credentials (normally regaining uid 0).

Related:
https://fedorahosted.org/sssd/ticket/2061

struct hbac_eval_req is defined in header file and it has attribute
request_time with type time_t, but header file "time.h" was not included.
It was not problem, because time.h was indirectly included by stdlib.h
(stdlib.h -> sys/types.h -> time.h) in implementation files,
but other platforms can have other dependencies among header files.