Moving to a sys.config configuration

Rather than configuring specific apps in many places (bin/logplex,
bin/devel_logplex, logplex_app.erl), configurations are moved to a
sys.config file that can be loaded by adding `-config sys` to the `erl`
exectuable, or loaded automatically when generating an OTP release.
Live upgrade for OOM branch if merged in for v69.2