setpgid/setpgid01.c: cleanup

Delete some useless comments.

Do some code re-arrangement and remove the 'if (pgid == pid)' check,
which is useless.

Some cleanup.

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>
setpgid/setpgid01.c: add pid = 0 and pgid = 0 test

Signed-off-by: Xiaoguang Wang <wangxg.fnst@cn.fujitsu.com>
Reviewed-by: Jan Stancek <jstancek@redhat.com>