drop impl for Process shouldn't wait()

[glandium]

My use-case is that i want to spawn a (detached) subprocess that survives the parent process. The equivalent of doing "command&" in a shell script.

Per IRC discussion:
forget() it when you're done with it?
it probably shouldn't wait in the destructor
should probably have a separate wait method
afaik we've been trying to avoid blocking destructors
glandium: it could be marked #[must_use] and only be destroyed with a wait or detach method, for example

[emberian]

cc @alexcrichton

[alexcrichton]

Closing as a dupe of #13854, but copying comments over to that one.