prism-auto: in nailgun mode, try to stop existing server first

Sometimes, if the prism-auto scripts gets interrupted, an existing
nailgun server is not properly shut down and might break a subsequent
prism-auto run.

prism/etc/scripts/prism-auto

@@ -741,6 +741,15 @@ def benchmarkPropListFile(propListFile):
         propFile = items[1].strip()
         benchmarkPropertiesFile(os.path.join(dir, propFile))
 
+# (Re-)start the nailgun server
+# We first try to stop an existing server (which should fail quickly if there is none running)
+def restartNailGunServer():
+    print("Stopping existing nailgun server, if it's running...")
+    subprocess.Popen([options.ngprism, "stop"]).wait()
+    print("Starting nailgun server...")
+    os.system(options.prismExec + " -ng &")
+    time.sleep(0.5)
+
 #==================================================================================================
 # Main program
 #==================================================================================================
@@ -803,8 +812,7 @@ if options.nailgun:
     if options.echo or options.echoFull:
         print(options.prismExec + " -ng &")
     else:
-        os.system(options.prismExec + " -ng &")
-        time.sleep(0.5)
+        restartNailGunServer();
 for arg in args:
     if os.path.isdir(arg):
         benchmarkDir(arg)