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

scripts/checksrc.pl

@@ -53,11 +53,15 @@ my %banfunc = (
     "gets" => 1,
     "strtok" => 1,
     "sprintf" => 1,
+    "snprintf" => 1,
     "vsprintf" => 1,
+    "vsnprintf" => 1,
+    "sscanf" => 1,
     "strcat" => 1,
     "strncat" => 1,
     "strncpy" => 1,
     "strtok_r" => 1,
+    "strtol" => 1,
     "strtoul" => 1,
     "_mbscat" => 1,
     "_mbsncat" => 1,