FpDbg: Refactored handling of breakpoints, so that parts of the code could be used too on other platforms then Windows
FpDbg: Set the FMainThread variable after the main-thread is created, not before. This fixes the handling of breakpoints.

git-svn-id: http://svn.freepascal.org/svn/lazarus/trunk@44445 4005530d-fff6-0310-9dd1-cebe43e6787f