* keygen.c (gen_elg, gen_dsa, gen_rsa, do_create, do_generate_keypair,

generate_subkeypair): New is_subkey argument to set whether a generated
key is a subkey.  Do not overload the ret_sk.  This is some early cleanup
to do backsigs for signing subkeys.

* keygen.c (write_keybinding, do_generate_keypair, generate_subkeypair):
Keep track of the unprotected subkey secret key so we can make a backsig
with it.

* keygen.c (make_backsig): New function to add a backsig to a binding sig
of signing subkeys.  Currently disabled. (write_keybinding): Call it here,
for signing subkeys only.

* sign.c (make_keysig_packet): Allow generating 0x19 signatures (same as
0x18 or 0x28, but used for backsigs).

* packet.h, build-packet.c (build_sig_subpkt): Add new SIGSUBPKT_SIGNATURE
type for embedded signatures.

g10/packet.h

@@ -364,6 +364,7 @@ typedef enum {
     SIGSUBPKT_SIGNERS_UID  =28, /* signer's user id */
     SIGSUBPKT_REVOC_REASON =29, /* reason for revocation */
     SIGSUBPKT_FEATURES     =30, /* feature flags */
+    SIGSUBPKT_SIGNATURE    =32, /* embedded signature */
 
     SIGSUBPKT_FLAG_CRITICAL=128
 } sigsubpkttype_t;