examples: harden systemd services

Add various settings to the example chronyd and chrony-wait services to
decrease the exposure reported by the "systemd-analyze security"
command. The original exposure was high as the analyzer does not check
the actual process (e.g. that it dropped the root privileges or that it
has its own seccomp filter).

Limit read-write access to /run, /var/lib/chrony, and /var/spool.
Access to /run (instead of /run/chrony) is needed for the refclock
socket expected by gpsd.

The mailonchange directive is most likely to break as it executes
/usr/sbin/sendmail, which can do unexpected operations depending on the
implementation. It should work with a setuid/setgid binary, but it is
not expected to write outside of /var/spool and the private /tmp.

examples/chrony-wait.service

@@ -16,5 +16,32 @@ TimeoutStartSec=180
 RemainAfterExit=yes
 StandardOutput=null
 
+CapabilityBoundingSet=
+DevicePolicy=closed
+DynamicUser=yes
+IPAddressAllow=localhost
+IPAddressDeny=any
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+PrivateDevices=yes
+PrivateUsers=yes
+ProcSubset=pid
+ProtectClock=yes
+ProtectControlGroups=yes
+ProtectHome=yes
+ProtectHostname=yes
+ProtectKernelLogs=yes
+ProtectKernelModules=yes
+ProtectKernelTunables=yes
+ProtectProc=invisible
+ProtectSystem=strict
+RestrictAddressFamilies=AF_INET AF_INET6
+RestrictNamespaces=yes
+RestrictRealtime=yes
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallFilter=~@privileged @resources
+UMask=0777
+
 [Install]
 WantedBy=multi-user.target