posix: Replace posix_spawnattr_tc{get,set}pgrp_np with posix_spawn_file_actions_addtcsetpgrp_np

The posix_spawnattr_tcsetpgrp_np works on a file descriptor (the
controlling terminal), so it would make more sense to actually fit
it on the file actions API.

Also, POSIX_SPAWN_TCSETPGROUP is not really required since it is
implicit by the presence of tcsetpgrp file action.

The posix/tst-spawn6.c is also fixed when TTY can is not present.

Checked on x86_64-linux-gnu and i686-linux-gnu.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>
Tested-by: Carlos O'Donell <carlos@redhat.com>