When a new process group is created, it is launched in the background
and any attempt to access the session terminal triggers a SIGTTIN (for
stdin) or SIGTTOU (for stdout) signal. These are ignored in an
interactive shell, but the default signal behavior in a new job is to
send a SIGTSTP to the whole process group. This causes runfvp to hang
when executed via a subprocess when stdin is accessed.

After creating a new process group, use tcsetpgrp to make the new group
the foreground process for the terminal associated with stdin/stdout,
but only if stdin is a tty.

The documentation for tcsetgrp states that tcsetpgrp itself raises a
SIGTTOU signal, so set this signal to SIG_IGN.

Signed-off-by: Peter Hoyes <Peter.Hoyes@arm.com>
Change-Id: I349a825df7fcb8a3cedb81762b901c6f50fa53b5
Signed-off-by: Jon Mason <jon.mason@arm.com>