Rename the local_subkey and remote_subkey fields in the auth_context

Rename the local_subkey and remote_subkey fields in the auth_context
to send_subkey and recv_subkey, respectively.  Add new APIs to query
and set these fields.  Change the behavior of mk_req_ext, rd_req_dec,
and rd_rep to set both subkeys.  Applications wanting to set
unidirectional subkeys may still do so by saving the values of subkeys
and doing overrides.  Cause mk_cred, mk_priv, and mk_safe to never use
the recv_subkey.  Cause rd_cred, rd_priv, and rd_safe to never use the
send_subkey.

ticket: 1415
status: open
tags: pullup
target_version: 1.3

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@15407 dc483132-0cff-0310-8789-dd5450dbe970