examples: set attr table

Florian adviced rtnl-route-event.c has same problem
examples: fix trivial error message
examples: use mnl_nlmsg_get_payload()

not increment (nlmsghdr) pointer
document: mnl_socket_open

fix return value on error
attr: validate MNL_TYPE_MSEC same as MNL_TYPE_U64
examples: made sub functions the same

show entry in oneline and RTA_PRIORITY
examples: fix display condition