expand bindir, libdir and mandir in the thisroot scripts so that they

can be used to set PATH, LD_LIBRARY_PATH and MANPATH. ROOTSYS itself 
is not used in prefix mode. Fixes issue 34935.


git-svn-id: http://root.cern.ch/svn/root/trunk@22913 27541ba8-7e3a-0410-8455-c3a389f83636