get_deckey and get_certkey

Note : get_certkey is for the secret part of the signing key
       get_sigkey if for the public part of the signing key
       renaming cert->sign and sig->valid would be less confusing

along with signature info and signature raw packet.

Effects :
 - Should import untrusted key data safely, keyring file/mem read replaced
   by validating filter. Only keys, uid or subkey with valid sig can enter.
 - Keyring queries use stored signature infos to elect best result. Uses
   youngest relevant signature to decide about expiration.
 - Support for key/subkey encryption and signing roles described in rfc4880
   through the Key Flags subpacket.

Side effects:
 - Encrypted seckey support broken
 - SSH keyring support broken
 - Probably some quircks in packet-print.c

Status:
 - Builds
 - Untested

Started extending pgp_key_t, changes need to be propagated.

Changed:
 pgp_subsig_t.pgp_sig_t -> pgp_uidsig_t.pgp_sig_info_t
 copy_sig_info : also duplicate sig's BNs
 pgp_validate_key_cb :
    - accepts NULL validate_key_cb_t,  in case result's
      siglists are discarded (i.e. for filtering).
    - relase packet eventualy copied by on_valid.

Renamed:
 pgp_key_t.subsigs -> pgp_key_t.uidsigs
 pgp_subsig_t -> pgp_uidsig_t