userauth: derive publickey from private

Pass a NULL pointer for the publickey parameter of
libssh2_userauth_publickey_fromfile and
libssh2_userauth_hostbased_fromfile functions.  In this case, the
functions recompute the public key from the private key file data.

This is work done by Jean-Louis CHARTON
<Jean-Louis.CHARTON@oikialog.com>, then adapted by Mark Smith and
slightly edited further by me Daniel.

WARNING: this does leave the feature NOT WORKING when libssh2 is built
to use libgcrypt instead of OpenSSL simply due to lack of
implementation.

src/openssl.h

@@ -241,3 +241,11 @@ int _libssh2_cipher_crypt(_libssh2_cipher_ctx * ctx,
 const EVP_CIPHER *_libssh2_EVP_aes_128_ctr(void);
 const EVP_CIPHER *_libssh2_EVP_aes_192_ctr(void);
 const EVP_CIPHER *_libssh2_EVP_aes_256_ctr(void);
+
+int _libssh2_pub_priv_keyfile(LIBSSH2_SESSION *session,
+                              unsigned char **method,
+                              size_t *method_len,
+                              unsigned char **pubkeydata,
+                              size_t *pubkeydata_len,
+                              const char *privatekey,
+                              const char *passphrase);