add support for safer algorithms than des-cbc-crc:v4 to asetkey

[Karl-Philipp Richter]

According to `man asetkey` only `des-cbc-crc:v4` algorithm is supported which is obsolete because it's considered insecure. The [OpenAFS quick start guide](http://docs.openafs.org/QuickStartUnix.pdf) recommends to not use it.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: openafs-krb5 1.6.15-1ubuntu1
ProcVersionSignature: Ubuntu 4.4.0-22.39-generic 4.4.8
Uname: Linux 4.4.0-22-generic x86_64
NonfreeKernelModules: openafs
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
Date: Sat May 14 23:34:42 2016
InstallationDate: Installed on 2015-04-20 (390 days ago)
InstallationMedia: Ubuntu-Server 14.10 "Utopic Unicorn" - Release amd64 (20141022.2)
ProcEnviron:
 TERM=screen.xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: openafs
UpgradeStatus: Upgraded to xenial on 2016-03-03 (72 days ago)