Trigger `RLogger` through environment variable

[Axel-Naumann]

Is your feature request related to a problem? Please describe.

It's not relevant for this feature request, but: this would allow CMS to trace autoparsing by setting an environment variable, after #7609 is merged.

Describe the solution you'd like

export ROOT_LOG='ROOT.InterpreterPerf=Debug(3),ROOT.RBrowser=Error'

should set verbosity for ROOT.InterpreterPerf to kDebug+3 and for ROOT.RBrowser to kError.

Describe alternatives you've considered

. in variable names is forbidden in bash, so export ROOT_LOG_ROOT.InterpreterPerf='Debug(3)' doesn't work, and we'd need to mangle this. That's ugly and complex.

We could also have an entry in .rootrc - but it brings in a dependency for RLogger on TEnv that I'd like to avoid.

Additional context

#6931

[Axel-Naumann]

This should also take gDebug into account.