Use unix.Prctl() instead of manually reimplementing it using
unix.RawSyscall. Also use unix.PR_SET_CHILD_SUBREAPER
unix.PR_GET_CHILD_SUBREAPER instead of locally defining them.
Also fix the package name form 'osutils' to 'sys' in the package level
comment.
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>