posix_spawn_wrapper: do not set POSIX_SPAWN_SETSIGDEF flag

posix_spawn_wrapper: do not set POSIX_SPAWN_SETSIGDEF flag

Setting this flag is a noop without a corresponding call to
posix_spawnattr_setsigdefault.

If we call posix_spawnattr_setsigdefault with a full signal set,
it causes glibc's posix_spawn implementation to call sigaction 63 times,
once for each signal. That seems wasteful.

This feature is really only useful for signals which have their
disposition set to SIG_IGN. Otherwise the dispostion gets set to
SIG_DFL automatically, either by clone(CLONE_CLEAR_SIGHAND) or the
subsequent execve.

As far as I can tell, systemd does not have any signals set to SIG_IGN
under normal operating conditions.