Generate systemd file from template and make installs it and the conf

The unit file Type=, as well as the executable path should now be correct
with a simple “make install”, whatever the install prefix and the DESTDIR
values are.

ref #3152

unit/biboumi.service.cmake

@@ -0,0 +1,15 @@
+[Unit]
+Description=Biboumi, XMPP to IRC gateway
+After=network.target
+
+[Service]
+Type=${SYSTEMD_SERVICE_TYPE}
+ExecStart=${CMAKE_INSTALL_PREFIX}/bin/biboumi /etc/biboumi/biboumi.cfg
+ExecReload=/bin/kill -s USR1 $MAINPID
+WatchdogSec=10
+Restart=always
+User=nobody
+Group=nobody
+
+[Install]
+WantedBy=multi-user.target