Fix daemon upgrade when running under systemd

Systemd assumes that service of type simple is stopped when the main
process exits. This causes systemd to kill all nghttpx processes when
doing a process upgrade (via USR2/QUIT signals). Change the service
type to forking which behaves correctly on upgrade.

contrib/nghttpx.service.in

@@ -3,8 +3,8 @@ Description=HTTP/2 experimental proxy
 After=network.target
 
 [Service]
-Type=simple
-ExecStart=@bindir@/nghttpx --errorlog-syslog
+Type=forking
+ExecStart=@bindir@/nghttpx --conf=/etc/nghttpx/nghttpx.conf --pid-file=/run/nghttpx.pid --daemon
 
 [Install]
 WantedBy=multi-user.target