JettyServer: Enable systemd socket activation for gerrit

Using a "gerrit.socket" unit file like this:

[Unit]
Description=Gerrit HTTP socket

[Socket]
ListenStream=80
Accept=no

[Install]
WantedBy=sockets.target

and a corresponding gerrit.service file enables an automatic start of gerrit
on the first request on port 80.

Change-Id: I1f82fed3c17f231915a5add4d7336a2b88457bc3

Documentation/config-gerrit.txt

@@ -2254,6 +2254,14 @@ to permit fast restarts.
 +
 By default, true.
 
+[[httpd.inheritChannel]]httpd.inheritChannel::
++
+If true, permits the daemon to inherit its server socket channel
+from fd0/1(stdin/stdout). When set to true, the server can be socket
+activated via systemd or xinetd.
++
+By default, false.
+
 [[httpd.requestHeaderSize]]httpd.requestHeaderSize::
 +
 Size, in bytes, of the buffer used to parse the HTTP headers of an

gerrit-pgm/src/main/java/com/google/gerrit/pgm/http/jetty/JettyServer.java

@@ -263,7 +263,7 @@ public class JettyServer {
       } catch (URISyntaxException e) {
         throw new IllegalArgumentException("Invalid httpd.listenurl " + u, e);
       }
-
+      c.setInheritChannel(cfg.getBoolean("httpd", "inheritChannel", false));
       c.setReuseAddress(reuseAddress);
       connectors[idx] = c;
     }

gerrit-pgm/src/main/resources/com/google/gerrit/pgm/systemd/gerrit.service

@@ -0,0 +1,17 @@
+# Systemd unit file for gerrit
+
+[Unit]
+Description=Gerrit Code Review
+After=syslog.target network.target
+
+[Service]
+Type=simple
+WorkingDirectory=/opt/gerritsrv/
+Environment=GERRIT_HOME=/opt/gerritsrv/gerrit/ JAVA_HOME=/opt/jdk1.8.0_45/
+ExecStart=/usr/bin/java -Xmx1024m -jar ${GERRIT_HOME}/bin/gerrit.war daemon -d ${GERRIT_HOME}
+User=gerritsrv
+SyslogIdentifier=GerritCodeReview
+StandardInput=socket
+
+[Install]
+WantedBy=multi-user.target

gerrit-pgm/src/main/resources/com/google/gerrit/pgm/systemd/gerrit.socket

@@ -0,0 +1,9 @@
+[Unit]
+Description=Gerrit HTTP socket
+
+[Socket]
+ListenStream=80
+Accept=no
+
+[Install]
+WantedBy=sockets.target