Re-implement the systemd support using the new query user API

Re-implement the systemd support using the new query user API

This provides exactly the same systemd functionality which existed
before the query user infrastructure got implemented.

  [v5 - Ensure NULL termination fix in d09fbf958f1c is included ]

  [v4 - change disapproved &= syntax ]

  [v3 - Remove QUERY_USER_EXEC_ALTERNATIVE macro, simplify
        alternatives definition directly in console.h.  For
        now only depend on ENABLE_SYSTEMD]

  [v2 - Removed the QUERY_USER_FOREACH macro]

Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Selva Nair <selva.nair@gmail.com>
Message-Id: 1470999445-4288-1-git-send-email-davids@openvpn.net
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg12424.html