ldpd: do not consume vty_conf when updating the configuration

ldpd: do not consume vty_conf when updating the configuration

David Lamparter gave the idea of keeping vty_conf as a permanent copy of
ldpd_conf in order to simplify the CLI code and facilitate the integration
with his cap'n proto framework in the future. Doing this demanded quite
some effort but it was worth it as the code looks much better now.

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>