A libabac keystore is a simple directory filled with some number of X509
identity and X509 attribute certificates. It may optionally include RSA
private keys.

X509 identity certs are identified as files whose names end in _ID.der
or _ID.pem. The part before the _ID.pem is irrelevant, though for the
sake of convneince the libabac credential manager gives it the name of
the CN you supplied. If the optional private key file is available, it
would be stored in _private.der or _private.pem with the same prefix as
the matching id file.

A new combined identity and private key cert format is implemented. 
It is a single file whose name ends in _IDKEY.der or _IDKEY.pem. 
The identity and private key informations are stored sequentially within
the file in any order. If either part is missing, libabac will handle 
accordingly and properly.

X509 attribute certs are identified as files whose names end in
_attr.der. The portion before the _attr.der is irrelevant.