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/test_keyboard_interactive_auth_fails_with_wrong_response.c

@@ -32,12 +32,12 @@ int test(LIBSSH2_SESSION *session)
     const char *userauth_list =
         libssh2_userauth_list(session, USERNAME,
                               (unsigned int)strlen(USERNAME));
-    if(userauth_list == NULL) {
+    if(!userauth_list) {
         print_last_session_error("libssh2_userauth_list");
         return 1;
     }
 
-    if(strstr(userauth_list, "keyboard-interactive") == NULL) {
+    if(!strstr(userauth_list, "keyboard-interactive")) {
         fprintf(stderr,
                 "'keyboard-interactive' was expected in userauth list: %s\n",
                 userauth_list);