When the v6 prefix is ::/0, then it is considered as less specific than
any v4-mapped prefix.

Remark that "no source prefix" is encoded by "src_plen == 0".  This
might be an issue, since "src_plen == 0" means ::/0, a v6 prefix.  So
the route (1.1.1.1/32 from 0.0.0.0/0) is internally represented by
(1.1.1.1/32 from ::/0).  When compared with  (::/0 from 2001:db8::/32),
the two source prefixes are comparable.  This is not a problem in
practice, since 2 pairs of prefixes are disjoint whether the dst
prefixes or the src prefixes are disjoint.

This allows 0 to appear within a channel list, which is consistent with
draft-chroboczek-babel-diversity-routing-01.

Noted by Matthieu Boutier.

Many of the directives don't make sense after startup.

The absolute path to the socket can be passed to the -g/-G command line
options, or to the new local-path/local-path-readwrite config file
directives.
Signed-off-by: Julien Cristau <jcristau@debian.org>

Now that the parser can be invoked at runtime, we need to be extra
careful with error handling.

Noted by Baptiste Jonglez.

Thanks to Dave Taht and Julien Cristau for pointing out the issue.

It's been broken for ages, nobody noticed.