Add extensive help to the .R command. Type .R -? or -h or --help to get

a detailed description of the options. Also support for sourced scripts
is added, as well as the old behaviour to execute a custom roots script,
to for example start in a special mode or special binary. To source a
script before starting roots do:

.R lxplus <go

to execute your own roots like script do:

.R lxplus myroots


git-svn-id: http://root.cern.ch/svn/root/trunk@21330 27541ba8-7e3a-0410-8455-c3a389f83636