Updated systemd files for linux (#6592)

Previous version put $BASE directory in root directory.
This version clearly explains how to run as root or as specific user.

Additional configuration:

* send SIGHUP for clean exit,

* restart on fail.

Tested on Ubuntu 16.04.3 LTS with 4.10.0-33-generic x86_64 kernel

scripts/parity.service

@@ -3,8 +3,18 @@ Description=Parity Daemon
 After=network.target
 
 [Service]
-EnvironmentFile=-%h/.parity/parity.conf
-ExecStart=/usr/bin/parity $ARGS
+# run as root, set base_path in config.toml
+ExecStart=/usr/bin/parity --config /etc/parity/config.toml
+# To run as user, comment out above and uncomment below, fill in user and group
+# picks up users default config.toml in $HOME/.local/.share/io.parity.ethereum/
+# User=username
+# Group=groupname
+# ExecStart=/usr/bin/parity
+Restart=on-failure
+
+# Specifies which signal to use when killing a service. Defaults to SIGTERM.
+# SIGHUP gives parity time to exit cleanly before SIGKILL (default 90s)
+KillSignal=SIGHUP
 
 [Install]
 WantedBy=default.target