Explain the "server is older than xxx warning".

* g10/call-agent.c (warn_version_mismatch): Print a note on how to
restart the servers.
* g10/call-dirmngr.c (warn_version_mismatch): Ditto.
* sm/call-agent.c (warn_version_mismatch): Ditto.
* sm/call-dirmngr.c (warn_version_mismatch): Ditto.

We should move this fucntion to common. However, the status output
functions are different and would need to be streamlined too.

• GnuPG-bug-id: T3117
• Debian-bug-id: #860745
• Signed-off-by: Werner Koch <wk@gnupg.org>