Make KCM the default ccache on OS X

If we do not find a default ccache value from krb5-config and we
detect that the host platform is OS X 10.7 or higher, use KCM: as the
default ccache name instead of FILE:/tmp/krb5cc_%{uid}.

ticket: 7964
Document KCM ccache type

Document the new KCM ccache type in ccache_def.rst.  Document the
kcm_socket and kcm_mach_service variables in krb5_conf.rst.

ticket: 7964