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

tests/server/tftpd.c

@@ -454,7 +454,7 @@ static ssize_t write_behind(struct testcase *test, int convert)
     c = *p++;                     /* pick up a character */
     if(prevchar == '\r') {        /* if prev char was cr */
       if(c == '\n')               /* if have cr,lf then just */
-        lseek(test->ofile, -1, SEEK_CUR); /* smash lf on top of the cr */
+        curl_lseek(test->ofile, -1, SEEK_CUR); /* smash lf on top of the cr */
       else if(c == '\0')          /* if have cr,nul then */
         goto skipit;              /* just skip over the putc */
       /* else just fall through and allow it */