checksrc: reduce directory-specific exceptions

By making them defaults, then fixing and/or reshuffling remaining
exceptions as necessary.

- checksrc: ban by default: `snprintf`, `vsnprintf`, `sscanf`, `strtol`.
- examples: replace `strtol` with `atoi` to avoid a checksrc exception.
- tests/libtest: replace `strtol` with `atol`.
- tests/server: replace most `strtol` with `atol`.
- tests/server: replace most `strtoul` with `atol`/`atoi`.
- tests/server: drop no longer used `util_ultous`.
- fix typo in checksrc rules: `vsnprint` -> `vsnprintf`.
- update local exceptions.

Also:
- examples: ban curl printf functions. They're discouraged in user code.
- examples: replace curl printf with system printf.
  Add `snprintf` workaround for <VS2015.
- examples/synctime: fix `-Wfloat-equal`.
- examples/synctime: exclude for non-Windows and non-UWP Windows.
- examples/synctime: build by default.

Closes #18823

docs/examples/http2-serverpush.c

@@ -33,6 +33,10 @@
 #include <curl/curl.h>
 #include <curl/mprintf.h>
 
+#if defined(_MSC_VER) && (_MSC_VER < 1900)
+#define snprintf _snprintf
+#endif
+
 #ifndef CURLPIPE_MULTIPLEX
 #error "too old libcurl, cannot do HTTP/2 server push!"
 #endif
@@ -173,7 +177,7 @@ static int server_push_callback(CURL *parent,
 
   (void)parent;
 
-  curl_msnprintf(filename, 128, "push%u", count++);
+  snprintf(filename, sizeof(filename), "push%u", count++);
 
   /* here's a new stream, save it in a new file for each new push */
   out = fopen(filename, "wb");