Make CBS_get_any_asn1_element() more compliant with DER encoding.

Make CBS_get_any_asn1_element() more compliant with DER encoding.

CBS_get_any_asn1_element violates DER encoding by allowing indefinite
form.  All callers except bs_ber.c expect DER encoding.  The callers
must check to see if it was indefinite or not.

Rather than exposing all callers to this behavior,
cbs_get_any_asn1_element_internal() allows specifying whether you want to
allow the normally forbidden indefinite form.  This is used by
CBS_get_any_asn1_element() for strict DER encoding and by a new static
function in bs_ber.c for the relaxed version.

While I was here, I added comments to differentiate between ASN.1
restrictions and CBS limitations.

ok miod@