[fpc-pascal] Function for checking correct file name

[Marco van de Voort]

In our previous episode, Graeme Geldenhuys said:
> > Right... but 255 characters for a filename is already rather much as
> > well when I compare this with the amount of text I can squash into a SMS ;)
> Now I can't remember exactly, is that 255 characters, or 255 bytes? 
> Because if it is the latter, then the Unicode character count limit 
> could be significantly less than 255.

Not really, since on Windows unicode means utf16