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

docs/examples/http2-upload.c

@@ -71,14 +71,14 @@ int my_gettimeofday(struct timeval *tp, void *tzp)
   (void)tzp;
   if(tp) {
     /* Offset between 1601-01-01 and 1970-01-01 in 100 nanosec units */
-    #define _WIN32_FT_OFFSET (116444736000000000)
+    #define WIN32_FT_OFFSET (116444736000000000)
     union {
       CURL_TYPEOF_CURL_OFF_T ns100; /* time since 1 Jan 1601 in 100ns units */
       FILETIME ft;
     } _now;
     GetSystemTimeAsFileTime(&_now.ft);
     tp->tv_usec = (long)((_now.ns100 / 10) % 1000000);
-    tp->tv_sec = (long)((_now.ns100 - _WIN32_FT_OFFSET) / 10000000);
+    tp->tv_sec = (long)((_now.ns100 - WIN32_FT_OFFSET) / 10000000);
   }
   return 0;
 }