tidy-up: example, tests continued

- fix skip auth if `userauthlist` is NULL.
  Closes #836 (Reported-by: @sudipm-mukherjee on github)
- fix most silenced `checksrc` warnings.
- sync examples/tests code between each other.
  (output messages, error handling, declaration order, comments)
- stop including unnecessary headers.
- always deinitialize in case of error.
- drop some redundant variables.
- add error handling where missing.
- show more error codes.
- switch `perror()` to `fprintf()`.
- fix some `printf()`s to be `fprintf()`.
- formatting.

Closes #960

tests/runner.c

@@ -41,7 +41,7 @@ int main(void)
 {
     int exit_code = 1;
     LIBSSH2_SESSION *session = start_session_fixture();
-    if(session != NULL) {
+    if(session) {
         exit_code = (test(session) == 0) ? 0 : 1;
     }
     stop_session_fixture();