Fix compressed io log corruption in background mode by using _exit()

instead of exit() to avoid flushing buffers twice.

Improved background mode support.  When not allocating a pty, the
command is run in its own process group.  This prevents write access
to the tty.  When running in a pty, stdin is not hooked up and we
never read from /dev/tty, which results in similar behavior.

--HG--
branch : 1.8