add untested systemd socket and service units

uffd-socketmap@.service

+[Unit]
+Description=Socketmap proxy for uffd mail alias lookup
+After=network.target
+Before=postfix.service
+
+[Service]
+ExecStart=/usr/bin/uffd-socketmap --socket-fd 3
+
+Restart=always
+RestartSec=10
+StandardOutput=syslog
+StandardError=syslog
+SyslogIdentifier=uffd-socketmap-%I
+
+PrivateUsers=true
+CapabilityBoundingSet=
+NoNewPrivileges=true
+RemoveIPC=true
+LockPersonality=true
+ProtectControlGroups=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectKernelLogs=true
+ProtectClock=true
+ProtectHostname=true
+ProtectProc=noaccess
+RestrictRealtime=true
+RestrictSUIDSGID=true
+RestrictNamespaces=true
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6
+ProtectSystem=strict
+ProtectHome=true
+PrivateTmp=true
+PrivateDevices=true
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+
+EnvironmentFile=/etc/uffd-socketmap/defaults
+EnvironmentFile=/etc/uffd-socketmap/$I.env
+
+[Install]
+WantedBy=default.target

uffd-socketmap@.socket

+[Unit]
+Description=Socket proxy for uffd mail alias lookup
+PartOf=socketmap-proxy@%i.service
+
+[Socket]
+ListenStream=/run/socketmap-proxy/%I.sock
+SocketUser=postfix
+SocketGroup=postfix
+SocketMode=0640
+
+[Install]
+WantedBy=sockets.target