Avoid leaving stdin in nonblocking mode after a modern prompt

Summary:
Ref T13649. Currently, "arc" may leave stdin nonblocking after showing a prompt. This can cause various odd behaviors down the line.

I can't immediately reproduce this behavior on macOS in "zsh" or "bash" (I'm unable to get stdin to remain nonblocking beyond the process lifespan), and also don't have pcntl locally so there's a fair amount of handwaving here.

Test Plan: This is somewhat speculative since I can't immediately reproduce the behavior. I tested the locally-reachable paths (no pcntl) but they're not interesting.

Maniphest Tasks: T13649

Differential Revision: https://secure.phabricator.com/D21666