Add and enable blackbox_exporter package

*blackbox_exporter* is a generic exporter for Prometheus that can be
used to provide metrics about HTTP, etc. services.  I use it to monitor
the availability and responsiveness of websites I host.

package/blackbox_exporter/blackbox_exporter.service

@@ -0,0 +1,45 @@
+[Unit]
+Description=Blackbox exporter
+Documentation=https://github.com/prometheus/blackbox_exporter/blob/master/README.md
+After=network-online.target
+Wants=network-online.target
+
+[Service]
+Type=exec
+ExecStart=/usr/bin/blackbox_exporter \
+	--config.file=/etc/prometheus/blackbox.yml \
+	--web.listen-address=[::1]:9115
+ExecReload=/bin/kill -HUP $MAINPID
+Restart=on-failure
+
+CapabilityBoundingSet=
+DeviceAllow=
+DevicePolicy=closed
+DynamicUser=yes
+LockPersonality=yes
+MemoryDenyWriteExecute=yes
+NoNewPrivileges=yes
+PrivateDevices=yes
+PrivateUsers=yes
+PrivateTmp=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
+RestrictSUIDSGID=yes
+SystemCallArchitectures=native
+SystemCallFilter=@system-service
+SystemCallFilter=~@privileged @resources
+UMask=0027
+
+[Install]
+WantedBy=multi-user.target