rootcint.cxx: For non-prefix build, rootcint uses its location to
determine ROOTSYS, now also on windows. Fixes ROOTSYS for rootcint_tmp.

TWinNTSystem.cxx: add a registry entry such that double clicking .root
files starts root and opens the file. Also sets icon and title for .root-files.

$ROOTSYS/etc/fileopen.C can be customized; currently it simply opens a
TBrowser. The registry is only touched if $ROOTSYS/macros/fileopen.C exists.


git-svn-id: http://root.cern.ch/svn/root/trunk@23972 27541ba8-7e3a-0410-8455-c3a389f83636