sim: print pid in output header

This information is useful if you have a bunch of simulations running
and want to know which one to kill, but you've neglected to take
advantage of the previous patch and embed the pid in your output path.

src/python/m5/main.py

@@ -291,7 +291,8 @@ def main(*args):
 
         print "gem5 started %s" % \
             datetime.datetime.now().strftime("%b %e %Y %X")
-        print "gem5 executing on %s" % socket.gethostname()
+        print "gem5 executing on %s, pid %d" % \
+            (socket.gethostname(), os.getpid())
 
         # in Python 3 pipes.quote() is moved to shlex.quote()
         import pipes