Darwin: set startup-with-shell to off on Sierra and later.

On Mac OS X Sierra and later, the shell is not allowed to be
debug so add a check and disable startup with shell in that
case. This disabling is done temporary before forking
inferior and restored after the fork.

gdb/ChangeLog:

        * darwin-nat.c (should_disable_startup_with_shell):
        New function.
        (darwin_nat_target::create_inferior): Add call.

Change-Id: Ie4d9090f65fdf2e83ecf7a0f9d0647fb1c27cdcc