use DispatchIO for Process.execute

[tanner0101]

This takes a shot at fixing #176 by using DispatchIO.

[vzsg]

One thing that wasn't mentioned in the other issue but in the Discord discussion that presumed it is that those pipes should probably be closed manually after the child process exited. Otherwise, there's a risk of running out of file descriptors when a Vapor app calls external processes often.

[tanner0101]

@vzsg yeah I think you're right. I've added code to close the read end of the pipes once the DispatchIO stream is closed.

[calebkleveter]

Is there a way to reliably fail the older version of Process.asyncExecute in a test case, so we can verify that this PR fixes it?

[tanner0101]

@calebkleveter I can't think of a way to do that reliably. :\ But maybe someone else does.

[tanner0101]

This fix is being held up by a bug with DispatchIO on Linux. I'm talking w/ Apple to see if there's anyway we can do about it.