osdep/unix/exec: Avoid atexit() handlers when child execvp() fails

osdep/unix/exec: Avoid atexit() handlers when child execvp() fails

The functions grub_util_exec_pipe() and grub_util_exec_pipe_stderr()
currently call execvp(). If the call fails for any reason, the child
currently calls exit(127). This in turn executes the parents
atexit() handlers from the forked child, and then the same handlers
are called again from parent. This is usually not desired, and can
lead to deadlocks, and undesired behavior. So, change the exit() calls
to _exit() calls to avoid calling atexit() handlers from child.

Fixes: e75cf4a58 (unix exec: avoid atexit handlers when child exits)
Signed-off-by: Dimitri John Ledkov <xnox@ubuntu.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>