Enable weak enctypes for aklog if supported by Kerberos

Enable weak enctypes for aklog if supported by Kerberos

Current versions of both MIT Kerberos and Heimdal disable DES enctypes
by default, but DES enctypes are still required for AFS service tickets.
Probe for either krb5_allow_weak_crypto() (MIT Kerberos 1.8) or
krb5_enctype_enable() (Heimdal) and, if found, call them to enable DES
enctypes.  If neither is found, assume that the Kerberos libraries are
old enough that DES is enabled by default.

Change-Id: Ib5231bb7c2fe88f4c424628394ed08122d5710a1
Reviewed-on: http://gerrit.openafs.org/1141
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>