More linker bloat reorganisation:

Split private key PEM and normal PEM handling. Private key
handling needs to link in stuff like PKCS#8.

Relocate the ASN1 *_dup() functions, to the relevant ASN1
modules using new macro IMPLEMENT_ASN1_DUP_FUNCTION. Previously
these were all in crypto/x509/x_all.c along with every ASN1
BIO/fp function which linked in *every* ASN1 function if
a single dup was used.

Move the authority key id ASN1 structure to a separate file.
This is used in the X509 routines and its previous location
linked in all the v3 extension code.

Also move ASN1_tag2bit to avoid linking in a_bytes.c which
is now largely obsolete.

So far under Linux stripped binary with single PEM_read_X509
is now 238K compared to 380K before these changes.