*: read and ignore srcdest routes on ZAPI

*: read and ignore srcdest routes on ZAPI

Since zebra might be sending srcdest routes down to the various daemons,
they need to understand the presence of the field at the very least.
Sadly, that's also the best we can do at this point since none of the
protocols has support for handling srcdest routes.  The only consistent
thing to do is to ignore them throughout.

If an administrator wants to have the srcdest route as non-srcdest in a
protocol, setting a non-srcdest static route (possibly blackhole) is
probably the best way to go.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>