Centralized the launching of a process into Target::Launch()

While investigating test suite failures when running the test suite remotely, I noticed we had 3 copies of code that launched a process:
1 - in "process launch" command
2 - SBTarget::Launch() with args
3 - SBTarget::Launch() with SBLaunchInfo

"process launch" was launching through the platform if it was supported (this is needed for remote debugging) and the 2 and 3 were not.

Now all code is in one place.