PATCH: Don't downcase filepath/filename while loading libraries

I just put this line in my postgresql.conf:

```
shared_preload_libraries = '/Path/Contains/UpCaseWords/an_ext.so'
```

Then the server couldn't start. It tried to load the file
"/path/contains/upcasewords/an_ext.so" and failed.

After few digging, I found there's a wrong use of `SplitIdentifierString`
in function `load_libraries` in /src/backend/utils/init/miscinit.c, and the
attached patch fixes it.

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