[tex-live] behaviour of kpsewhich on case-insensitive mac system

[Akira Kakuto]

> It would be wrong to "randomly" return any case-insensitive match 

I have changed to return actual file names on Windows,
probably with a little loss of performance.

In TeX Live 2018, users will obtain:

C:\Users\Philip.Taylor>kpsewhich PLAIN.TEX
c:/TeX/Live/2018/texmf-dist/tex/plain/base/plain.tex

C:\Users\Philip.Taylor>kpsewhich PLAIN.TeX
c:/TeX/Live/2018/texmf-dist/tex/plain/base/plain.tex

C:\Users\Philip.Taylor>kpsewhich Plain.TeX
c:/TeX/Live/2018/texmf-dist/tex/plain/base/plain.tex

C:\Users\Philip.Taylor>kpsewhich plain.TeX
c:/TeX/Live/2018/texmf-dist/tex/plain/base/plain.tex

C:\Users\Philip.Taylor>kpsewhich ieeetran.cls
c:/TeX/Live/2018/texmf-dist/tex/latex/ieeetran/IEEEtran.cls

Thanks,
Akira