std::run loops forever if exec fails

[thestinger]

We should just be using posix_spawn here, both for performance and to avoid these edge cases.

[alexcrichton]

This works both in the native and the libuv cases now (not sure what ended up fixing this).

Flagging as needstest, although all of the process tests need to be run on 1:1 and M:N. I'm making a framework for this soon after #10965 lands, so I'll take care of this soon after that.

[alexcrichton]

A test is now checked in (https://github.com/mozilla/rust/blob/b7f673a627c674b289721ee339fd2980919c6afd/src/libstd/io/process.rs#L200-L213), closing.