tidy-up: avoid using the reserved macro namespace

To avoid hitting `-Wreserved-macro-identifier` where possible.

- amigaos: introduce local macro instead of reusing `__request()`.
- easy_lock: avoid redefining `__has_builtin()`.
  Follow-up to 33fd57b8ff #9062
- rand: drop interim macro `_random()`.
- windows: rename local macro `_tcsdup()` to `Curl_tcsdup()`.
  To avoid using the reserved macro namespace and to avoid
  colliding with `_tcsdup()` as defined by Windows headers.
- checksrc: ban `_tcsdup()` in favor of `Curl_tcsdup()`.
- tool_doswin: avoid redefining `_use_lfn()` (MS-DOS).
- tool_findfile: limit `__NO_NET_API` hack to AmigaOS.
  Syncing this pattern with `lib/netrc.c`.
  Follow-up to 784a8ec2c1 #16279
- examples/http2-upload: avoid reserved namespace for local macro.

More cases will be removed when dropping WinCE support via #17927.

Cases remain when defining external macros out of curl's control.

Ref: #18477
Closes #18482

lib/memdebug.h

@@ -48,11 +48,11 @@
 #define recv(a,b,c,d) curl_dbg_recv(a,b,c,d, __LINE__, __FILE__)
 
 #ifdef _WIN32
-#undef _tcsdup
+#undef Curl_tcsdup
 #ifdef UNICODE
-#define _tcsdup(ptr) curl_dbg_wcsdup(ptr, __LINE__, __FILE__)
+#define Curl_tcsdup(ptr) curl_dbg_wcsdup(ptr, __LINE__, __FILE__)
 #else
-#define _tcsdup(ptr) curl_dbg_strdup(ptr, __LINE__, __FILE__)
+#define Curl_tcsdup(ptr) curl_dbg_strdup(ptr, __LINE__, __FILE__)
 #endif
 #endif /* _WIN32 */