launch-client, launcher-service: Use a pseudo-terminal to enable interactive TUI debugging

• launcher-service: Try to set the controlling terminal

  If stdin, stdout or stderr is a terminal, we want to set it as our controlling terminal to get things like job control.

• utils: Move functions for a hash table keyed by struct stat to here

  A subsequent commit will need these.

• pty-bridge: Add a bridge between fds and a pseudo-terminal

  This can be fd-passed across a container boundary to stop programs in the container from tampering with the real terminal outside the container, while providing proper job control for programs inside the container.

• launch-client: Use a pseudo-terminal to replace references to terminals

  This allows the program on the service side to take the pseudo-terminal as its controlling terminal, giving it proper job control.

  steamrt/tasks#125

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Still to do:

• test SIGWINCH handling
• test TIOCSTI blocking