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

lib/curl_setup.h

@@ -490,28 +490,28 @@
 #  include <sys/types.h>
 #  include <sys/stat.h>
    /* Large file (>2Gb) support using Win32 functions. */
-#  undef  lseek
-#  define lseek(fdes, offset, whence)  _lseeki64(fdes, offset, whence)
 #  undef  fstat
-#  define fstat(fdes, stp)             _fstati64(fdes, stp)
-#  define struct_stat                  struct _stati64
-#  define LSEEK_ERROR                  (__int64)-1
+#  define fstat(fdes, stp)                _fstati64(fdes, stp)
+#  define struct_stat                     struct _stati64
+#  define curl_lseek                      _lseeki64
+#  define LSEEK_ERROR                     ((__int64)-1)
 #elif defined(__DJGPP__)
    /* Requires DJGPP 2.04 */
 #  include <unistd.h>
-#  undef  lseek
-#  define lseek(fdes, offset, whence)  llseek(fdes, offset, whence)
-#  define LSEEK_ERROR                  (offset_t)-1
+#  define curl_lseek                      llseek
+#  define LSEEK_ERROR                     ((offset_t)-1)
+#elif defined(__AMIGA__)
+#  define curl_lseek(fd, offset, whence)  lseek(fd, (off_t)(offset), whence)
+#  define LSEEK_ERROR                     ((off_t)-1)
+#else
+#  define curl_lseek                      lseek
+#  define LSEEK_ERROR                     ((off_t)-1)
 #endif
 
 #ifndef struct_stat
 #define struct_stat struct stat
 #endif
 
-#ifndef LSEEK_ERROR
-#define LSEEK_ERROR (off_t)-1
-#endif
-
 #ifndef SIZEOF_TIME_T
 /* assume default size of time_t to be 32 bits */
 #define SIZEOF_TIME_T 4