Implementing ConcatFileName and Which to not return a new char[]
to be deleted by the user, but operate on an existing TString.
because of backward compatibility issues I had to introduce new function
names.

   Which(...const char*,...) -> FindFile(...TString&,...)
   ConcatFileName(...const char*,...) -> PrependPath(...TString&,...)

The existing Which and ConcatFilename are now wrappers for the new
TString versions. The wrapping is done in TSystem, so all derived
classes need to implement FindFile and PrependPath now.
I've also fixed a few obvious problems and incompatibilities between the
windows and the unix versions; the VMS version is IMHO still broken. One
of the problems fixed: ConcatFileName(0,"name") returned "name" on unix,
but "\\name" on windows. It now returns "name" for all.


git-svn-id: http://root.cern.ch/svn/root/trunk@15688 27541ba8-7e3a-0410-8455-c3a389f83636