Fix a bug preventing the pid file from being empty.

Convert os.getpid() to a string before writing it to the pid file in
order to avoid generating an empty pid file.

offlineimap/init.py

@@ -133,7 +133,7 @@ def startup(versionno):
 
     try:
         pidfd = open(config.getmetadatadir() + "/pid", "w")
-        pidfd.write(os.getpid())
+        pidfd.write(str(os.getpid()) + "\n")
         pidfd.close()
     except:
         pass