'Error: no valid primary key self-signature' importing private key

Hi,

Similar problem to #1270 and #1701 various others.

I've been trying to load a private key using kbpgp and I'm getting stuck at the "Error: no valid primary key self-signature".

I tested via "Host an encrypted copy of my private key" and got the same error; I've also tested with another more recent key and it works fine.

It's an ancient key (created: 2001-09-25), self signed, no expiry, and otherwise works perfectly within GPG. (I was able to load the public key as a KeyManager with no issues.)

Output of list-packets is below (Gpg4win v2.3.0):

```
:secret key packet:
version 4, algo 17, created 1001452531, expires 0
skey[0]: [1024 bits]
skey[1]: [160 bits]
skey[2]: [1022 bits]
skey[3]: [1024 bits]
iter+salt S2K, algo: 3, simple checksum, hash: 2, salt: 1df98601c32731fd
protect count: 65536 (96)
protect IV: 97 73 d5 ba 5f 04 ba 59
encrypted stuff follows
keyid: 9877B59888157914
:user ID packet: "David Harrison "
:user ID packet: "David Harrison "
:user ID packet: "David Harrison "
:secret sub key packet:
version 4, algo 16, created 1001452531, expires 0
skey[0]: [2048 bits]
skey[1]: [2 bits]
skey[2]: [2045 bits]
iter+salt S2K, algo: 3, simple checksum, hash: 2, salt: 01774fe610e51fe0
protect count: 65536 (96)
protect IV: aa 71 41 89 e9 32 a2 ff
encrypted stuff follows
keyid: EF51CB14ACF8C834
```

Thanks,

Start by reproducing the private-key import failure and the same error from “Host an encrypted copy of my private key,” using the supplied GPG list-packets output and a newer key for comparison. Done means determining whether the 2001 self-signed key can be imported successfully or documenting the unsupported key characteristic and its limitation.

Written by the indexing model from the issue text.