Implement a new module command that is called right before cosing the virtual machine to allow modules to release resources manually.

Should fix a possible crash in the IRC plugin which was caused by the Session destructor to be called recursively when cleaned automatically by the VM.

shared/SqMod.h

@@ -58,6 +58,7 @@ extern "C" {
     #define SQMOD_INITIALIZE_CMD    0xDABBAD00
     #define SQMOD_LOAD_CMD          0xDEADBABE
     #define SQMOD_TERMINATE_CMD     0xDEADC0DE
+    #define SQMOD_CLOSING_CMD       0xBAAAAAAD
     #define SQMOD_RELEASED_CMD      0xDEADBEAF
     #define SQMOD_API_VER           1