gpg: Print better debug info in case of broken sig subpackets.

* g10/parse-packet.c (enum_sig_subpkt): Print a hexdump.

A test key for this is 02DF08F5FD356BF27F5F7B838921B5DCCD15A883 .
A key listing in verbose mode will now yield

gpg: DBG: buffer shorter than subpacket (10/9/25);\
     dump: 19100d87e54973647cff

The error here is that the packet length is 0x19 but what follows is a
long keyid (subpacket 16) which is shorter. The debug output might be
helpful to better analyze broken signatures.

• Signed-off-by: Werner Koch <wk@gnupg.org>