bug#6784: 24.0.50; cmdproxy incosistency with command pathnames

[Laimonas Vėbra]

Óscar Fuentes wrote:

> That's one more incosistency: a long command works fine, then you put
> that command as part of a pipe chain and it stops working. I guess the
> current cmdproxy approach is the lesser evil.

It's a CreateProcess() (in cmdproxy.c) _valid_ path requirement/problem; 
valid path/directory separator is (should be) '\':

http://msdn.microsoft.com/en-us/library/aa365247(VS.85).aspx

(only File I/O API converts '/' to '\')

p.s. cmd.exe /c accepts both variants.