Implement an opt-in performance mode for the Posix process driver

[s-ludwig]

Should use waitpid(-1, ...) directly to wait for the next exited child instead of going through the full list of known processes. Since this potentially breaks third party code that works with processes, this cannot be the default. See also #122 (comment)

[Panke]

Since waitpid(-1, …) on it's own has the problem to interfere with processes started outside of vibe.d, we might think about moving all sub-processes into a distinct process group, which can be signaled and monitored separately from all others processes.