fix delv when using the builtin trust-anchors

fix delv when using the builtin trust-anchors

Since the builtin trust-anchors are now called `builtin-trust-anchors`,
delv needs specific handling in order to be able to parse those when
they are used.

Before, delv was simply parsing a single clause (either in the case of
an overriden trust-anchors value from bindkeys file or by simply reading
the builtin value). But since the name changed, the same code can't be
shared and the builtin version is expected to be in a map.