Use a pty for prefixed output instead of pipes for stdout/stderr. This
is so that programs can get information about the controlling terminal.
This change was triggered by bug #188506 where it's possible that
stdin, stdout and stderr didn't point to a terminal but ended up on one
via our pipes. Using a pty means that stdout and stderr always point to
a terminal, but we lose the ability to tell them apart.
If there is not a pty available then we use un-prefixed output as normal.
This change has also introduced the need for a signal pipe so that
SIGCHLD can exit the loop cleanly.