Closing a file from the wrong thread is harmless on most systems.

(See ticket #1611)
But on systems like RedHat9 with broken fcntl() locks, it leaks file
descriptors.  That is better than the alternative of prematurely breaking
locks and causing database corruption.  Nevertheless, it would be good
if we could figure out a way to report errors when closing a file from
the wrong thread. (CVS 2946)

FossilOrigin-Name: ad8f12cad1fcd9c54478abe20050f9a953035506