Unbreak DSA signature trailer handling

Unbreak DSA signature trailer handling
- RFC 4880 states the following:
	V4 signatures also hash in a final trailer of six octets: the
	version of the Signature packet, i.e., 0x04; 0xFF; and a four-octet,
	big-endian number that is the length of the hashed data from the
	Signature packet (note that this number does not include these final
	six octets).
  ... but we were using size_t, whose size is  platform dependent, oops :)