[SUREFIRE-949] add forkCount parameter

[SUREFIRE-949] add forkCount parameter, making the inconsitent forkMode parameter deprecated.

- All defaulting works as in the previous versions, with the exception of reuseForks (introduced in the last release). It's now true by default.
- forkCount supports "C" notation as in -T of maven-core

(I had some problems with building the latest head revision, so I branched from my previous work on SUREFIRE-946 - there's a separate pull-request for that one).