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

docs/internals/CODE_STYLE.md

@@ -332,6 +332,7 @@ makes us write better code.
 This is the full list of functions generally banned.
 
     _access
+    _lseeki64
     _mbscat
     _mbsncat
     _open
@@ -367,6 +368,7 @@ This is the full list of functions generally banned.
     getaddrinfo
     gets
     gmtime
+    llseek
     LoadLibrary
     LoadLibraryA
     LoadLibraryEx
@@ -374,6 +376,7 @@ This is the full list of functions generally banned.
     LoadLibraryExW
     LoadLibraryW
     localtime
+    lseek
     malloc
     mbstowcs
     MoveFileEx