Provide a single entry point for configuration

Looking at MNG-7772, this should not require any code change, but it's all about configuration.

I propose to load / interpolate the following files:

• ${maven.home}/conf/maven.user.properties
• ${maven.home}/conf/maven.system.properties

Those files would be used to load additional user properties and system properties for Maven.  In addition to the simple interpolation mechanism, we should provide two enhancements using special keys ${includes} and ${optionals} which would be used to load additional referenced configuration files such as:

    ${optionals} = ${user.home}/.m2/maven.user.properties, ${session.rootDirectory}/.mvn/maven.user.properties

Being loaded early when Maven is loaded, those files could reference directories to load extensions from:

  maven.core.extensions.directories = ${session.rootDirectory}/.mvn/extensions.xml,${user.home}/.m2/extensions.xml,${maven.home}/extensions.xml

In various places, the maven code could be simplified and offer more configuration points at the same time.