dmd: file: Add PROT_READ to mmap protections.

[yelninei]

On Hurd missing PROT_READ causes a segmentation fault with a later memcpy to the mmaped memory

As the file is opened with O_RDWR i dont think this should change anything.

I also changed a check for mmap failure to MAP_FAILED

Let me know if I should split that into 2 commit and if the commit message is ok.

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