ssh signing: use sigc struct to pass payload

ssh signing: use sigc struct to pass payload

To be able to extend the payload metadata with things like its creation
timestamp or the creators ident we remove the payload parameters to
check_signature() and use the already existing sigc->payload field
instead, only adding the length field to the struct. This also allows
us to get rid of the xmemdupz() calls in the verify functions. Since
sigc is now used to input data as well as output the result move it to
the front of the function list.

 - Add payload_length to struct signature_check
 - Populate sigc.payload/payload_len on all call sites
 - Remove payload parameters to check_signature()
 - Remove payload parameters to internal verify_* functions and use sigc
   instead
 - Remove xmemdupz() used for verbose output since payload is now already
   populated.

Signed-off-by: Fabian Stelzer <fs@gigacodes.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>