Flag user specified routes with an AF_LINK gateway as RTF_LLINFO.

Flag user specified routes with an AF_LINK gateway as RTF_LLINFO.

This makes ND6 and ARP layers happy since they both check for this flag,
but is was redundant to provide route(8)'s "-llinfo" option when adding
a l2 route.

RTF_LLINFO is clearly redundant for userland tools, so only allow the
kernel to set it.

ok benno@, claudio@