setpgid03: handle children errors and cleanup

If one of the children exits with non zero value we can't see
it until DEBUG is set. Now if the exit value is not 0, we
print this value with tst_resm(TFAIL).

Also performed minor cleanup.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
setpgid03: use tst_checkpoint interface

I came accross the following rare issue:

setpgid03    1  TPASS  :  setpgid SUCCESS to set errno to EPERM
setpgid03    2  TFAIL  :  setpgid FAILED, expect EACCES got 1

I suppose it may happen if the test host is under stress.

To prevent this it would be better to use tst_checkpoint interface
for parent-child synchronisations.

Signed-off-by: Stanislav Kholmanskikh <stanislav.kholmanskikh@oracle.com>
setpgid03: Further fixes.

Remove the alarm(), timeouts of checkpoint interface shouldn't be
handled in tests. These should either be handled in the checkpoint
interface or in the test driver.

Check retval from setpgid() properly.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
config.mk.in: Turn on extra warnings.

Don't forget to regenerate and rerun the configure script.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
doc: Fix PATH in test-writing-guidelines.txt

If test does tst_tmpdir() the working directory will be changed, we must
use PATH="$PATH:$PWD" instead.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>