[Lazarus] Reworked ReadFileToString to support Linux virtual files

[AlexeyT]

https://bugs.freepascal.org/view.php?id=36145

Last attached demo contains my reworked function (after the fix it works 
pretty fast, i tested it). Can you apply at least this function? The 
issue can be opened - issue is about IDE. The function is reworked to 
support Linux virtual files like /proc/version, /proc/meminfo.

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Regards,
Alexey