[MNG-6768] Support inheritable multiple settings

**[Marco Brandizi](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=zakmck)** opened **[MNG-6768](https://issues.apache.org/jira/browse/MNG-6768?redirect=false)** and commented

As far as I know, the --settings option supports just one settings file. Worse, when it is used, user settings in ~/.m2/settings.xml are ignored (not tried, but I suspect global settings are too).

To me, this is rather poor. It would be much better if
1. user and global settings were inherited and possibly overridden by the further settings file.
2. multiple --settings \ could be specified and a merge of settings could be composed by maven (eg, \ blocks would come from multiple files, \ would be overridden, considering the order in the command line).

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No further details from [MNG-6768](https://issues.apache.org/jira/browse/MNG-6768?redirect=false)