By introducing a new option -a in addition to -p this patch allows
controlling the on-link flag of announcements.

A prefix specified using -a will have the on-link flag set to zero
while a prefix specified using -p will retain its behaviour (i.e.
on-link flag set).

Example:

    gluon-radvd -i local-node -p 2001:db8:aaaa:/64 -a 2001:db8:bbbb::/64

This will announce 2001:db8:aaaa::/64 with the on-link flag set and
2001:db8:bbbb::/64 with the flag unset.

Moving the scripts to a common directory not only vastly simplifies the
zzz-gluon-upgrade script, but also allows to define an ordering of such
scripts across packages.

The specification says that the ifa_addr field of struct ifaddrs may be NULL,
and in Barrier Breaker, this does actually occur...

This reverts commit f5c532a3a923053ab229952bde31da0b0284d8ab.