Improve OSGi-friendliness of Log4j Core

While Log4j Core does support OSGi deployments, it is less OSGi-friendly than Logback.

As an indication of missing features, the PAX Logging project must override 6 classes from `log4j-core`, but only 1 class from Logback.

PAX Logging maintainer @grgrzybek, provided an extensive explanation of each override in [this long answer](https://github.com/apache/logging-log4j2/pull/1820#issuecomment-1784573202). We should probably evaluate the possibility of enhancing some of those classes.