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_info_request.c

@@ -223,7 +223,7 @@ LIBSSH2_ALLOC_FUNC(test_alloc)
 {
     int *threshold_int_ptr = *abstract;
     alloc_count++;
-    if(*abstract != NULL && *threshold_int_ptr == alloc_count) {
+    if(*abstract && *threshold_int_ptr == alloc_count) {
         return NULL;
     }
 
@@ -252,7 +252,7 @@ int test_case(int num,
     alloc_count = 0;
     free_count = 0;
     session = libssh2_session_init_ex(test_alloc, test_free, NULL, abstract);
-    if(session == NULL) {
+    if(!session) {
         fprintf(stderr, "libssh2_session_init_ex failed\n");
         return 1;
     }