vssh: keyfile use cleanups

- make the libssh backend do the same fallback for private and public key
  filename as libssh2 already did and is documented behavior. This now uses a
  common function.

- set the file names already in setup_connection if public key auth is
  requested, so that the connection reuse functions can use the
  information when checking for existing connections to reuse

- rename the oddly named struct fields 'rsa' to 'priv_key' and 'rsa_pub'
  to 'pub_key' to better reflect their purposes

Fixes #22243
Closes #22244

lib/vssh/vssh.h

@@ -29,6 +29,8 @@
 
 #include "urldata.h"
 
+struct ssh_conn;
+
 CURLcode Curl_getworkingpath(struct Curl_easy *data,
                              const char *homedir,
                              char **path);
@@ -40,5 +42,7 @@ CURLcode Curl_ssh_range(struct Curl_easy *data,
                         curl_off_t *startp, curl_off_t *sizep);
 
 CURLcode Curl_ssh_pollset(struct Curl_easy *data, struct easy_pollset *ps);
+CURLcode Curl_ssh_setup_pkey(struct Curl_easy *data, struct ssh_conn *sshc);
+
 #endif /* USE_SSH */
 #endif /* HEADER_CURL_VSSH_VSSH_H */