[SI-6502] fixed :cp command

by recreating interpreter for :cp command instead of merely resetting

Could not figure out how to the reset so that the compiler classpath is updated properly too.
Simply recreating (as was done before) does the job too though and all tests pass ... except the one for this commit that is ...

**NOT READY TO MERGE**

The test succeeds when I run `./test/partest test/files/run/cp-cmd/cp-cmd.scala` but fails with `./test/partest test/files/run/cp-cmd` which is what happens when the overall test suite is run, though. **Any ideas why that is?** The output is definitely correct (no difference between the run log and the check file).

**PathSetting**

PathSetting's `append` did not work correctly. Had to fix that to make the test pass as well.
The way it was implemented the `MutableSettings.SettingValue#postSetHook` was not triggered anyway which is unexpected behaviour IMO.