[MNG-6623] Adding / Removing from core multiplier for parallel builds

**[Jason Faust](https://issues.apache.org/jira/secure/ViewProfile.jspa?name=jftsunami)** opened **[MNG-6623](https://issues.apache.org/jira/browse/MNG-6623?redirect=false)** and commented

An extension to the parallel build syntax would be useful to fine tune the result after the first adjustment happens.

One idea is `nC(+|-)x` where `nC` is the existing syntax, and the plus or minus would increase or decrease the result, or another way:
* `1C+4` = Number of cores, plus 4 more threads, so on a 1 core system, 5 threads
* `2C-2` = On a 4 core system, 6 threads

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**Affects:** 3.6.0