Add support for variable-length session keys in our client code.

Add support for variable-length session keys in our client code.

This means that we now support 'net rpc join' with KRB5 (des based)
logins.  Now, you need to hack 'net' to do that, but the principal is
important...

When we add kerberos to 'net rpc', it should be possible to still do
user management and the like over RPC.

(server-side support to follow shortly)

Andrew Bartlett
(This used to be commit 9ecf9408d98639186b283f1acf0fac46417547d0)