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Related to changes from previous patch.

In some circumstances during reconfiguration, routes propagated by pipes
to other tables may hang there even after the primary routes are removed.

There is already a workaround for this issue in the code which removes
these stale routes by flush process when source protocols are shut down.

This patch is a cleaner fix and allows to simplify the flush process

Linux kernel route metrics (RTA_METRICS netlink route attribute) are
represented and accessible as new route attributes:

krt_mtu, krt_window, krt_rtt, krt_rttvar, krt_sstresh, krt_cwnd, krt_advmss,
krt_reordering, krt_hoplimit, krt_initcwnd, krt_rto_min, krt_initrwnd,
krt_quickack, krt_lock_mtu, krt_lock_window, krt_lock_rtt, krt_lock_rttvar,
krt_lock_sstresh, krt_lock_cwnd, krt_lock_advmss, krt_lock_reordering,
krt_lock_hoplimit, krt_lock_rto_min, krt_feature_ecn, krt_feature_allfrag

New LSA checksumming code separates generic Fletcher-16 and OSPF-specific
code and avoids back and forth endianity conversions, making it much more
readable and also several times faster.

Prior to this patch, BIRD validates the OSPF LSA checksum by calculating
a new checksum and comparing it with the checksum in the header. Due to
the specifics of the Fletcher checksum used in OSPF, this is not
necessarily correct as the checkbytes in the header may be calculated via
a different means and end up with a different value that is nonetheless
still correct.

The documented means of validating the checksum as specified in RFC 905
B.4 is to calculate c0 and c1 from the unchanged contents of the packet,
which must result in a zero value to be considered valid.

Thanks to Chris Boot for the patch.

Did not really worked

When a new route was imported from kernel and chosen as preferred, then
the old best route was propagated as a withdraw to the kernel protocol.
Under some circumstances such withdraw propagated to the BSD kernel could
remove the new alien route and thus reverting the import.

The bug caused that received external LSAs with locally reachable
next hops were ignored. I wonder why nobody noticed it sooner.

Under some circumstances and heavy load, TX could be postponed
until the session fails with hold timer expired.

Thanks to Javor Kliachev for making the bug reproductible.

Unfortunately, some interfaces support multicast but do not have
this flag set, so we use it only as a positive hint.

Thanks to Clint Armstrong for noticing the problem.

RFC 6286 relaxed rules for router IDs, allowing EBGP sessions between
routers with the same ID (but different ASN).

Also hook feed_done is renamed to feed_end.

Temporary rta is reused in BGP, while rta_lookup() breaks it.

Thanks to Alexander Chernikov for analysing the problem.

Using 'dump sockets' in IPv6 mode caused crash due to mismatched format string.

Thanks to Pavel Tvrdik for noticing it.

Also update log message for error-triggered startup delay.

Permit specifying neighbor address, AS number and port independently.
Add 'interface' parameter for specifying interface for link-local
sessions independently.

Thanks to Alexander V. Chernikov for the original patch.

This value is specified in documentation.

Make proto_config_new() use this info instead of supplied size.

Thanks to Alexander V. Chernikov for the patch.

Thanks to Alexander V. Chernikov for the patch.

Thanks to Pavel Tvrdik for noticing it.

Thanks to Alexander Chernikov for the patch.