Add --no-set-foreground option to parrot to execute the tracee in the background.

Currently, parrot allows the tracee to set its process group ID to its own PID and
set its process group as the foreground process group of the session by default,
which may break the program which first calls parrot and then needs to use
terminal (to use terminal, the program must be in the foregroud process group).
Merge remote-tracking branch 'ccl/pull/651/head'

* ccl/pull/651/head:
  Add --no-set-foreground option to parrot to execute the tracee in the background.