tidy-up: avoid using the reserved macro namespace

[vszakats]

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 33fd57b easy_lock: fix the #ifdef conditional for ia32_pause #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 784a8ec tidy-up: delete, comment or scope C macros reported unused #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