Add a new control argument to TMemStat

// When TMemStat is active it recors every call to malloc/free in a ROOT Tree.
// You must be careful when running jobs with many millions (or more) of calls
// to malloc/free because the generated Tree may become very large.
// The TMemStat constructor TMemStat(const char* system, Int_t buffersize, Int_t maxcalls)
// has its 3 arguments optional:
//   -system refers to the internal algorithm to compute the back traces.
//    the recommended value is "gnubuiltin"
//   -buffersize is the number of calls to malloc or free that can be stored in one memory buffer.
//    when the buffer is full, the calls to malloc/free pointing to the same location
//    are eliminated and not written to the final Tree. The default value 100000
//    is such that between 50 and 90% of the calls are eliminated depending on the application.
//    You can set buffersize <=1 to keep every single call to malloc/free.
//   -maxcalls can set a limit for the maximum number of calls to be registered in the Tree.
//    The default value is 5000000.
// The 3 arguments can be set  in $ROOTSYS/etc/system.rootrc
//    Root.TMemStat.system      gnubuiltin
//    Root.TMemStat.buffersize  100000
//    Root.TMemStat.maxcalls    5000000


git-svn-id: http://root.cern.ch/svn/root/trunk@36382 27541ba8-7e3a-0410-8455-c3a389f83636