Retry upon NET_RT_IFLIST sysctl failure.

Port the logic used by getifaddrs(3) to handle the case where
NET_RT_IFLIST returns ENOMEM, which can occur if the list size changes
between the buffer allocation and sysctl read.

PR: 195191
Submitted by: Guy Yur <guyyur@gmail.com> (original version)
MFC after: 1 week