[startup] fixMayapy2011SegFault - allow manual invoking

if it's called twice, won't double-install overrides (this makes manually
calling it allowable)

This still doesn't guarantee we will get the correct exit code in all
situations, as it's still possible for callers to get the original
(unwrapped) versions before they are wrapped

pymel/internal/startup.py

@@ -416,6 +416,7 @@ def uninitializeMayaOnPythonExit():
         atexit.register(uninitializeMayaOnPythonExit)
         _atExitCallbackInstalled = True
 
+_fixMayapy2011SegFault_applied = False
 
 # Have all the checks inside here, in case people want to insert this in their
 # userSetup... it's currently not always on
@@ -425,6 +426,12 @@ def fixMayapy2011SegFault():
     import platform
     if platform.system() == 'Linux':
         if om.MGlobal.mayaState() == om.MGlobal.kLibraryApp:  # mayapy only
+            global _fixMayapy2011SegFault_applied
+            if _fixMayapy2011SegFault_applied:
+                return
+
+            _fixMayapy2011SegFault_applied = True
+
             # In linux maya 2011, once maya has been initialized, if you try
             # to do a 'normal' sys.exit, it will crash with a segmentation
             # fault..