[dnssec] keys: abstract storage from PEM

dnssec/Makefile.am

dnssec/lib/dnssec/key.h

dnssec/lib/dnssec/keystore.h

dnssec/lib/key/convert.c

dnssec/lib/key/convert.h

dnssec/lib/key/dnskey.c

dnssec/lib/key/dnskey.h

dnssec/lib/key/internal.h

dnssec/lib/key/key.c

dnssec/lib/key/keyid.c

dnssec/lib/key/simple.c

dnssec/lib/keystore/keystore.c

dnssec/lib/keystore/pkcs8.c

dnssec/lib/keystore/public.c

dnssec/lib/keystore/public.h

dnssec/tests/keystore_pkcs8.c