reenable Int3 debugging
check for GM_MULTI before trying to do network jobs
Enable Int3 as d_debugbreak

Change Int3 from a no-op to a platform-specific trap to debugger.

Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/pull/18
Merge commit 'Re-enable debug breaks' into unification/master

Requested by btb: https://github.com/dxx-rebirth/dxx-rebirth/pull/18