Rewrite PKCS#12 code and remove some of the old

Rewrite PKCS#12 code and remove some of the old
horrible macros.

Fix two evil ASN1 bugs. Attempt to use 'ctx' when
NULL if input is indefinite length constructed
in asn1_check_tlen() and invalid pointer to ASN1_TYPE
when reusing existing structure (this took *ages* to
find because the new PKCS#12 code triggered it).