Refactor asn1_d2i_ex_primitive()

Refactor asn1_d2i_ex_primitive()

Split the object content handling off into asn1_d2i_ex_primitive_content(),
move the handling ov V_ASN1_ANY into asn1_d2i_ex_any() and move the MSTRING
handling into asn1_d2i_ex_mstring(). This way we parse the header once
(rather than twice for ANY and MSTRING), then process the content, while
also avoiding complex special cases in a single code path.

ok tb@