-Open socket() at first and then setuid() to actual user.
-Allow ping6 preload option only for root.
Approved by: jkh
Submitted by: Neil Blakey-Milner <nbm@mithrandr.moria.org>
(1)added error check of if_nameindex() return value at getaddrinfo().
(2)print out more detailed information when getaddrinfo() error value
is EAI_SYSTEM.(in this case system error num is kept in errno)
(1) is Discovered by: jinmei@kame.net in KAME environment.
