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

@@ -3,8 +3,8 @@ Description=Biboumi, XMPP to IRC gateway
 After=network.target
 
 [Service]
-Type=notify
-ExecStart=/usr/bin/biboumi /etc/biboumi/biboumi.cfg
+Type=${SYSTEMD_SERVICE_TYPE}
+ExecStart=${CMAKE_INSTALL_PREFIX}/bin/biboumi /etc/biboumi/biboumi.cfg
 ExecReload=/bin/kill -s USR1 $MAINPID
 WatchdogSec=10
 Restart=always