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/rand.c

@@ -149,12 +149,6 @@ static CURLcode weak_random(struct Curl_easy *data,
 }
 #endif
 
-#ifdef USE_SSL
-#define _random(x,y,z) Curl_ssl_random(x,y,z)
-#else
-#define _random(x,y,z) weak_random(x,y,z)
-#endif
-
 static CURLcode randit(struct Curl_easy *data, unsigned int *rnd,
                        bool env_override)
 {
@@ -185,7 +179,11 @@ static CURLcode randit(struct Curl_easy *data, unsigned int *rnd,
 #endif
 
   /* data may be NULL! */
-  return _random(data, (unsigned char *)rnd, sizeof(*rnd));
+#ifdef USE_SSL
+  return Curl_ssl_random(data, (unsigned char *)rnd, sizeof(*rnd));
+#else
+  return weak_random(data, (unsigned char *)rnd, sizeof(*rnd));
+#endif
 }
 
 /*