(svn r1131) -Add: [Network] Autoclean_companies (set it with 'set autoclean_companies on/off').

When enabled, empty companies (companies with no active clients) with no 
 password are declared bankrupt after 1 year of emptyness.
For empty companies with password, the password is removed after 3 years 
 of emptyness.
The delay of removing company/password can be configured via:
 - 'set autoclean_protected <months>'
 - 'set autoclean_unprotected <months>'

network_server.h

@@ -14,6 +14,7 @@ void NetworkServer_HandleChat(NetworkAction action, DestType desttype, int dest,
 
 bool NetworkServer_ReadPackets(ClientState *cs);
 void NetworkServer_Tick();
+void NetworkServerMonthlyLoop();
 
 #endif /* ENABLE_NETWORK */