lib: make yang_dnode_get_entry() more flexible

lib: make yang_dnode_get_entry() more flexible

Add the "abort_if_not_found" parameter to the yang_dnode_get_entry()
function instead of always aborting when an user pointer is not
found.  This will make it possible, for example, to use this function
during the validation phase of a configuration transaction. Callers
will only need to check if the function returned NULL or not,
since new configuration objects (if any) won't be created until
the NB_EV_APPLY phase of the transaction.

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