lib, rip, ripng, babel, eigrp: add ctx pointer to distribute api

lib, rip, ripng, babel, eigrp: add ctx pointer to distribute api

a distribute_ctx context pointer is returned after initialisation to the
calling daemon. this context pointer will be further used to do
discussion with distribute service. Today, there is no specific problem
with old api, since the pointer is the same in all the memory process.
but the pointer will be different if we have multiple instances. Right
now, this is not the case, but if that happens, that work will be used
for that.
distribute-list initialisation is split in two. the vty initialisation
is done at global level, while the context initialisation is done for
each routing daemon instance.
babel daemon is being equipped with a routing returning the main babel
instance.
also, a delete routine is available when the daemon routing instance is
suppressed.
a list of contexts is used inside distribute_list. This will permit
distribute_list utility to handle in the same daemon to handle more than
one context. This will be very useful in the vrf context.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>