Support packages that are head-signed
This supports packages which do not have the RPMv3 signature scheme
(over the full RPM header+payload), but instead only have signatures
over the header (v4 scheme).
For the v4 scheme, the signature is only stored in SIGTAG_RSA (or
SIGTAG_DSA).
Signed-off-by: Patrick Uiterwijk <patrick@puiterwijk.org>