main: conditionalize fixup_environment() internally

main: conditionalize fixup_environment() internally

This code is executed before we parse command line/configuration
parameters, hence let's not use arg_system to figure our how to clean up
things, but instead PID == 1. Let's move that check inside of the
function, to make things a bit more robust abstract from the outside.

Also, let's add a log message about this, that was so far missing.