zebra: add pseudowire manager

zebra: add pseudowire manager

Base framework for supporting MPLS pseudowires in FRR.

A consistent zserv interface is provided so that any client daemon
(e.g. ldpd, bgpd) can install/uninstall pseudowires in a standard
way. Static pseudowires can also be implemented by using the same
interface.

When zebra receives a request to install a pseudowire and the installation
in the kernel or hardware fails, a notification is sent back to the
client daemon and a new install attempt is made every 60 seconds (until
it succeeds).

Support for external dataplanes is provided by the use of hooks to
install/uninstall pseudowires.

Signed-off-by: ßingen <bingen@voltanet.io>
Signed-off-by: Renato Westphal <renato@opensourcerouting.org>