gpg: Make --export-ssh-key work for the primary key.

* g10/export.c (export_ssh_key): Also check the primary key.
--

If no suitable subkey was found for export, we now check whether the
primary key is suitable for export and export this one.  Without this
change it was only possible to export the primary key by using the '!'
suffix in the key specification.

Also added a sample key for testing this.

GnuPG-bug-id: 2957
Signed-off-by: Werner Koch <wk@gnupg.org>

tests/openpgp/samplekeys/README

@@ -17,3 +17,5 @@ E657FB607BB4F21C90BB6651BC067AF28BC90111.asc Key with subkeys (no protection)
 rsa-rsa-sample-1.asc   RSA+RSA sample key (no passphrase)
 ed25519-cv25519-sample-1.asc  Ed25519+CV25519 sample key (no passphrase)
 silent-running.asc     Collection of sample secret keys (no passphrases)
+rsa-primary-auth-only.pub.asc  rsa2408 primary only, usage: cert,auth
+rsa-primary-auth-only.sec.asc  Ditto but the secret keyblock.

tests/openpgp/samplekeys/rsa-primary-auth-only.pub.asc

@@ -0,0 +1,23 @@
+pub   rsa2048 2017-02-14 [CA]
+      F74B4029E6906D12EBDA8EE3BD7744900FDABC8D
+      Keygrip = AB1BB1843677AF7CC4D6C14444320C3FF4147E98
+uid           [ unknown] ssh://host.example.net
+
+-----BEGIN PGP PUBLIC KEY BLOCK-----
+
+mQENBFiizWgBCACi28riS0AaC7UvXaZfoafEvcXq/MAq6akiowPf3eY4zz5DkBPf
+Ep3kGuDMAFqULvchIt9vpg719Zar/Xldi+UG+/KsDz+TT5k+nP6CwvBHbAXXtISv
+S51TKKnTFpvjcgJc1BMFN0pGf7JnZx1QfRfsZO2BvS4qVzYCWbSS9hlpMq4aIgOc
+ERBMsZYMPnI4ijbXysksecDC91kbJH0q5j8aGir5sDyrDwfVLp0SUAubRFU5gXuZ
+SEv9QmeV7XoXKXzk9KEYy7GUgoAJzabvbF0rVXqd3DE8KFkwK7rKBe8sGC04DWlK
+j/sHJcAfMSqCi/SZyYpO+FSfnB+uJ1BNc05hABEBAAG0FnNzaDovL2hvc3QuZXhh
+bXBsZS5uZXSJAU4EEwEIADgWIQT3S0Ap5pBtEuvajuO9d0SQD9q8jQUCWKLNaAIb
+IQULCQgHAgYVCAkKCwIEFgIDAQIeAQIXgAAKCRC9d0SQD9q8jZBrB/41MJWUeclV
+tM60+ydPNgUJwyRXpKdLIm/AtM1zOijlFkwsaMmzbFSFJJX98HGASHdU5OpL2Lv3
+1NNDNMbUuFumApVrLzJUBugFRb+8/uY7H3Z0/YKQ9g9OC3z7+uqFFv/+/wA+VdYX
+Zy6uim8E4OlJ41S68fQcMiTxbLTCDkvBbpf505t6JhNqF6JB+SBFQJXvRqjoydXf
+dyoiDz9N1V0ERzmGEiPewvHg2zWcVia07NGhxN3slQ3klOfYJQ8Ye72feNq1zKCy
+AyU3X8fL10XKWooCAU+t4hR5hXYxYTSZse5q0FHZ38Lt9c3ApMSZ2+ueeOtGbsH9
+kV8icGkI6KXp
+=zMXp
+-----END PGP PUBLIC KEY BLOCK-----

tests/openpgp/samplekeys/rsa-primary-auth-only.sec.asc

@@ -0,0 +1,38 @@
+sec   rsa2048 2017-02-14 [CA]
+      F74B4029E6906D12EBDA8EE3BD7744900FDABC8D
+      Keygrip = AB1BB1843677AF7CC4D6C14444320C3FF4147E98
+uid           [ unknown] ssh://host.example.net
+
+Passprase: none
+
+-----BEGIN PGP PRIVATE KEY BLOCK-----
+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+=3QG9
+-----END PGP PRIVATE KEY BLOCK-----