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

scripts/checksrc.pl

@@ -49,6 +49,7 @@ my @ignore_line;
 
 my %banfunc = (
     "_access" => 1,
+    "_lseeki64" => 1,
     "_mbscat" => 1,
     "_mbsncat" => 1,
     "_open" => 1,
@@ -84,6 +85,7 @@ my %banfunc = (
     "getaddrinfo" => 1,
     "gets" => 1,
     "gmtime" => 1,
+    "llseek" => 1,
     "LoadLibrary" => 1,
     "LoadLibraryA" => 1,
     "LoadLibraryEx" => 1,
@@ -91,6 +93,7 @@ my %banfunc = (
     "LoadLibraryExW" => 1,
     "LoadLibraryW" => 1,
     "localtime" => 1,
+    "lseek" => 1,
     "malloc" => 1,
     "mbstowcs" => 1,
     "MoveFileEx" => 1,