--- change_privilege(id)

    /¸/¸桼 ID Τ٤Ƥ¾Υ桼ˤʤޤ
     id ֤ޤ

    Ǥʤ(桼 ID ѹǤʤ)㳰 
    ArgumentError ȯޤ㳰ȯˤΥ᥽åɤƤ
    Фγƥ桼 ID ͤ¸Ƥ뤫ɤݾڤޤ

    Process.uid= ȤϸߴʤȤդƤ
    setreuid(id,-1) ˤäƼƤΤѤƤϡ
    мΤ褦˽񤭴ɬפޤ

        Process::UID.re_exchange  # (r,e,s)==(u1,u2,??) ==> (u2,u1,??)
        Process::UID.eid = id     # (u2,u1,??) ==> (u2,id,??)
        Process::UID.re_exchange  # (u2,id,??) ==> (id,u2,??)

