lib: fix a couple of yang validation issues

lib: fix a couple of yang validation issues

libyang-0.16-r3 contains a commit[1] that changed the autodelete
behavior of subtrees when validating data. A few FRR commands were
affected by this change since they relied on the old autodelete
behavior.

To fix these commands, use the LYD_OPT_WHENAUTODEL flag when
validating data to restore the old autodelete behavior (which adds
a lot of convenience for us).

[1] https://github.com/CESNET/libyang/commit/bbc43b1b4

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