Apply an exclusive lock in VFileOpenFD if opening the file as writable

[CasualPokePlayer]

Untested, should prevent situations such as #3580 from occurring.

[endrift]

Two suggestions:

1. Let's put this in the map function if it's MAP_WRITE
2. Check the advisory lock on Unix while when opening

[CasualPokePlayer]

1. I assume this would end up meaning we should unlock it at unmap. Which then leads to the question, how do we know if the unmap is with a read-only mapping or a writable mapping? I would guess each mapping would need to be tracked, for both Windows and POSIX platforms (Granted, the API seems inconsistent in whether or not it expects multiple mappings to occur for the same VFile, the Windows code implies that is possibly expected given its HandleMappingList vector, although the POSIX path with _POSIX_MAPPED_FILES undefined ends up having a writable member that's set on each _vfdMap call?)
2. I assume this just means replacing F_SETLK with F_GETLK and such and responding accordingly?