Better handling of global addresses as configured NBMA neighbors in OSPFv3.

Configured NBMA neighbors in OSPFv3 should be link-local addresses, old
behavior was to silently ignore global ones. The patch allows BIRD to
accept global ones, but adds a warning and a documentation notice.

Thanks to Wilco Baan Hofman for the bugreport.
Implements 'next hop keep' option for BGP.

This option allows to keep the received next hop even in cases when
the route is sent to an interface with a different subnet.
Fixes a bug in IPv6 BGP next hop processing.

BGP next hop attributes with empty link-local IPv6 addresses were not
handled properly.

Thanks to Sergey Popovich for the bugfix.
Allows IP of loopback to be used in automatic router ID selection.

Thanks to Alexander V. Chernikov for the patch.
Fixes a compatibility issue in OSPFv2 PtP links.

BIRD used zero netmask in hello packets on all PtP links, not just on
unnumbered ones. This patch fixes it and adds option 'ptp netmask'
for overriding the default behavior.

Thanks to Alexander V. Chernikov for the original patch.
Adds two new default GCC options.

Adds two new default GCC options related to optimizations
(-fno-strict-aliasing and -fno-strict-overflow). This
should fix some hyperaggressive GCC optimizations.

Also updates autoconf option detection.
Merge branch 'birdcl'
Fixes a crash when mrtdump is enabled and interface goes away.

Thanks to Peter Christensen for the bugfix.
NEWS and version update.
Fixes a problem with BGP neighbors, link-local addresses and locking.

Thanks to Fritz Grimpen for the bugfix.
Fixes problems with routing table scans on some platforms.

Negative bit shifts are definitely undefined oprations.
Fixes crash with vlinks.