systemd: tighten the service security a bit

What's left enabled:

* Access to /dev -- obviously
* CAP_SYS_ADMIN -- this is needed by TIOCSSERIAL only. Too bad this also
  allows TIOCSTI, which allows for code injection unless something else
  (SELinux) disallows access to ttys with shells.
  Maybe kernel should use CAP_SYS_TTY_CONFIG for this.
* socket(AF_NETLINK) -- udev & kernel device changes
* socket(AF_UNIX) -- D-Bus

data/ModemManager.service.in

@@ -8,6 +8,12 @@ BusName=org.freedesktop.ModemManager1
 ExecStart=@sbindir@/ModemManager
 StandardError=null
 Restart=on-abort
+CapabilityBoundingSet=CAP_SYS_ADMIN
+ProtectSystem=true
+ProtectHome=true
+PrivateTmp=true
+RestrictAddressFamilies=AF_NETLINK AF_UNIX
+NoNewPrivileges=true
 
 [Install]
 WantedBy=multi-user.target