KRB5: Move ccache-related functions to krb5_ccache.c
    
    Add a new module krb5_ccache.c that contains all ccache-related
    operations. The only user of this module shall be krb5_child.c as the
    other modules will run unprivileged and accessing the ccache requires
    either privileges of root or the ccache owner.
    
    Related:
    https://fedorahosted.org/sssd/ticket/2370
    
    Reviewed-by: Sumit Bose <sbose@redhat.com>
    Reviewed-by: Lukáš Slebodník <lslebodn@redhat.com>