gluon-radvd: on-link flag

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.