Support systemd socket activation (#35)

* Support systemd socket activation

If the process has been started with systemd socket activation
configured, then serve requests on the passed-in socket instead of
attempting to bind to an address.

* Add example systemd unit files.

Add example systemd unit files which make use of systemd's security
facilities, and also allow binding to port 80 whilst running as an
unpriviliged process (using systemd socket activation).

systemd/speedtest-settings.toml

@@ -0,0 +1,31 @@
+# bind address, use empty string to bind to all interfaces, or when using socket activation
+#bind_address=""
+# backend listen port.  Set this to "" when using socket activation
+listen_port=""
+# proxy protocol port, use 0 to disable
+proxyprotocol_port=0
+# Server location
+server_lat=50.82589
+server_lng=-0.141391
+# ipinfo.io API key, if applicable
+ipinfo_api_key=""
+
+# assets directory path, defaults to `assets` in the same directory
+assets_path="/usr/local/share/speedtest/assets"
+
+# password for logging into statistics page
+statistics_password="PASSWORD"
+# redact IP addresses
+redact_ip_addresses=false
+
+# database type for statistics data, currently supports: none, memory, bolt, mysql, postgresql
+# if none is specified, no telemetry/stats will be recorded, and no result PNG will be generated
+#database_type="bolt"
+database_type="memory"
+database_hostname=""
+database_name=""
+database_username=""
+database_password=""
+
+# if you use `bolt` as database, set database_file to database file location
+database_file="/usr/local/var/speedtest/speedtest.db"