ssh-proxy: add ssh ProxyCommand tool that can connect to AF_UNIX + AF_VSOCK sockets

This adds a tiny binary that is hooked into SSH client config via
ProxyCommand and which simply connects to an AF_UNIX or AF_VSOCK socket
of choice.

The syntax is as simple as this:

     ssh unix/some/path     # (this connects to AF_UNIX socket /some/path)

or:

     ssh vsock/4711

I used "/" as separator of the protocol ID and the value since ":" is
already taken by SSH itself when doing sftp. And "@" is already taken
for separating the user name.

tmpfiles.d/20-systemd-ssh-generator.conf.in

@@ -0,0 +1,10 @@
+#  This file is part of systemd.
+#
+#  systemd is free software; you can redistribute it and/or modify it
+#  under the terms of the GNU Lesser General Public License as published by
+#  the Free Software Foundation; either version 2.1 of the License, or
+#  (at your option) any later version.
+
+# See tmpfiles.d(5) for details
+
+L {{SSHCONFDIR}}/20-systemd-ssh-proxy.conf - - - - {{LIBEXECDIR}}/ssh_config.d/20-systemd-ssh-proxy.conf

tmpfiles.d/meson.build

@@ -35,6 +35,7 @@ in_files = [['etc.conf',                      ''],
             ['systemd.conf',                  ''],
             ['var.conf',                      ''],
             ['20-systemd-userdb.conf',        'ENABLE_USERDB'],
+            ['20-systemd-ssh-generator.conf', ''],
            ]
 
 foreach pair : in_files