Allow complete control over stdio

[JordanMartinez]

Allows user to specify all of the stdio values for stdio[0..2] and reserver stdio[3] for IPC or null.

The goal here is to address purescript/spago#1048. We learned that piping the parent process' stdin into the child process stdin does not cause the child to terminate when user inputs Ctrl+D. The only workaround was to use inherit as the child process' stdio[0] value.