lxc/attach: Revert "- LXC attach should exit on SIGCHLD"

lxc/attach: Revert "- LXC attach should exit on SIGCHLD"

This reverts commit f02158439677d0c1d4b2ed2ed1ba9bc43923a05d.

Let's revert this change as it introduces 2 regressions:
1. it's not correct to do exit(2) from a signal handler in this case,
as we skip a proper cleaning procedures like restoring PTY configuration
state (see lxc_terminal_delete()) which leads to a problem with a PTY after lxc-attach exits.

[ hint: just try to use lxc-attach on a main branch with this change and you will
see it. After lxc-attach exits you won't be able to type anything in your
current terminal session as it's messed up. ]

2. this introduces race-condition in the code which leads to a
regression on LXD/(and I believe Incus too) which can be seen as
random "Failed to retrieve PID of executing child process" errors
on "lxc exec"/"incus exec" commands. It's extremely hard to reproduce,
but my guess is that we are getting a race condition here, because
by the time when we set a new signal handler for SIGCHLD, transient process
is still alive and when it exists it generates SIGCHLD which may lead to
exit().

3. This changes a behavior of lxc-attach which was there for *years*
and it's quite scary to be honest. I'm not against having this change, but
in a different form, for example we can add a new command line parameter for
lxc-attach command which will enable this behavior.

My first attempt was to fix that change to prevent race, but then
I've noticed that we also have a more serious problem described in (1),
this requires more work to do.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>