enable history file size management. History file size can now be specified
in rootrc file. Like:

# History file size, once HistSize is reached remove all but HistSave entries,
# set to 0 to turn off command recording.
# Can be overridden by environment variable ROOT_HIST=size[:save],
# the ":save" part is optional.
# Rint.HistSize:         500
# Set to -1 for sensible default (80% of HistSize), set to 0 to disable history.# Rint.HistSave:         400


git-svn-id: http://root.cern.ch/svn/root/trunk@16350 27541ba8-7e3a-0410-8455-c3a389f83636