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

src/tool_doswin.c

@@ -46,9 +46,10 @@
 #  undef  PATH_MAX
 #  define PATH_MAX MAX_PATH
 #elif !defined(__DJGPP__) || (__DJGPP__ < 2)  /* DJGPP 2.0 has _use_lfn() */
-#  define _use_lfn(f) (0)  /* long filenames never available */
+#  define CURL_USE_LFN(f) 0  /* long filenames never available */
 #elif defined(__DJGPP__)
-#  include <fcntl.h>       /* _use_lfn(f) prototype */
+#  include <fcntl.h>         /* for _use_lfn(f) prototype */
+#  define CURL_USE_LFN(f) _use_lfn(f)
 #endif
 
 #ifdef MSDOS
@@ -314,7 +315,7 @@ static SANITIZEcode msdosify(char **const sanitized, const char *file_name,
     return SANITIZE_ERR_INVALID_PATH;
 
   /* Support for Windows 9X VFAT systems, when available. */
-  if(_use_lfn(file_name)) {
+  if(CURL_USE_LFN(file_name)) {
     illegal_aliens = illegal_chars_w95;
     len -= (illegal_chars_w95 - illegal_chars_dos);
   }

src/tool_findfile.c

@@ -24,10 +24,14 @@
 #include "tool_setup.h"
 
 #ifdef HAVE_PWD_H
+#ifdef __AMIGA__
 #undef __NO_NET_API /* required for AmigaOS to declare getpwuid() */
+#endif
 #include <pwd.h>
+#ifdef __AMIGA__
 #define __NO_NET_API
 #endif
+#endif
 
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>