Tab completion, stacked redirection, unredirect to pipe, platform independence.

Tab completion works using the changed (but ROOT 5 compatible) syntax .> A.txt.
.> now uses dup2() that exists on Windows, too, instead of ttyname().
Tnis enables unredirection even during root > out.txt - which did not work in ROOT5.
Multiple ".> X.txt" can be stacked; ".>" unredirects back to the previous one on the stack.