Maven shell
Changes:
* (unrelated) contains fix for property handling in embedded executor and in lookup invoker
* pulled `-o` (offline) option to "generic" Options from MavenOptions
* introduce mvnsh (scripts, options, CLIng main class)
* simplified invokers (only one context needed for maven now)
* invokers made "reentrant", it all depends HOW context is created
Related PRs:
* mvnd changes https://github.com/apache/maven-mvnd/pull/1228
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https://issues.apache.org/jira/browse/MNG-8437