refactored credential builder

allow enumeration of matching builders
	try a second builder if the first one fails
	builder clones resources internally on demand
	caller frees added resources on failure and success
	stricter handling of non-supported build parts

src/charon/plugins/sql/sql_cred.c

@@ -64,7 +64,7 @@ static bool private_enumerator_enumerate(private_enumerator_t *this,
 	while (this->inner->enumerate(this->inner, &type, &blob))
 	{
 		this->current = lib->creds->create(lib->creds, CRED_PRIVATE_KEY, type,
-										   BUILD_BLOB_ASN1_DER, chunk_clone(blob),
+										   BUILD_BLOB_ASN1_DER, blob,
 										   BUILD_END);
 		if (this->current)
 		{
@@ -150,7 +150,7 @@ static bool cert_enumerator_enumerate(cert_enumerator_t *this,
 	while (this->inner->enumerate(this->inner, &type, &blob))
 	{
 		this->current = lib->creds->create(lib->creds, CRED_CERTIFICATE, type,
-										   BUILD_BLOB_ASN1_DER, chunk_clone(blob),
+										   BUILD_BLOB_ASN1_DER, blob,
 										   BUILD_END);
 		if (this->current)
 		{