build: tidy up local lseek() mappings

- stop redefining system symbol `lseek`, by introducing `curl_lseek()`.
- handle AmigaOS quirk within the macro mapping.
- add missing parenthesis to `LSEEK_ERROR` values.
- tool_util: use curl `lseek` macros in `tool_ftruncate64()`.
- move `LSEEK_ERROR` to right-hand side of if expressions.
- checksrc: disallow direct uses of `_lseeki64`, `llseek`, `lseek`.

Closes #20488

src/tool_util.c

@@ -86,7 +86,7 @@ int tool_ftruncate64(int fd, curl_off_t where)
 {
   intptr_t handle = _get_osfhandle(fd);
 
-  if(_lseeki64(fd, where, SEEK_SET) < 0)
+  if(curl_lseek(fd, where, SEEK_SET) == LSEEK_ERROR)
     return -1;
 
   if(!SetEndOfFile((HANDLE)handle))