Change type of offset arg in H5Pset_external to HDoff_t

The `off_t` type is only 32-bit on Windows, which makes it impossible to link to higher offsets in large files.

The `H5O_efl_entry_t` struct defines its `offset` field already as `HDoff_t`, so no additional conversion is needed.

bin/trace

@@ -109,6 +109,7 @@ $Source = "";
                "H5M_iterate_t"              => 'MI',
                "H5FD_mem_t"                 => "Mt",
                "off_t"                      => "o",
+               "HDoff_t"                    => "Ho",
                "H5O_iterate1_t"             => "Oi",
                "H5O_iterate2_t"             => "OI",
                "H5O_mcdt_search_cb_t"       => "Os",

src/H5Pdcpl.c

@@ -2599,7 +2599,7 @@ H5Pget_chunk_opts(hid_t plist_id, unsigned *options /*out*/)
  *-------------------------------------------------------------------------
  */
 herr_t
-H5Pset_external(hid_t plist_id, const char *name, off_t offset, hsize_t size)
+H5Pset_external(hid_t plist_id, const char *name, HDoff_t offset, hsize_t size)
 {
     size_t          idx;
     hsize_t         total, tmp;
@@ -2608,7 +2608,7 @@ H5Pset_external(hid_t plist_id, const char *name, off_t offset, hsize_t size)
     herr_t          ret_value = SUCCEED; /* Return value */
 
     FUNC_ENTER_API(FAIL)
-    H5TRACE4("e", "i*soh", plist_id, name, offset, size);
+    H5TRACE4("e", "i*sHoh", plist_id, name, offset, size);
 
     /* Check arguments */
     if (!name || !*name)

src/H5Ppublic.h

@@ -5570,6 +5570,21 @@ H5_DLL herr_t H5Pget_mpi_params(hid_t fapl_id, MPI_Comm *comm, MPI_Info *info);
  */
 H5_DLL herr_t H5Pset_mpi_params(hid_t fapl_id, MPI_Comm comm, MPI_Info info);
 #endif /* H5_HAVE_PARALLEL */
+
+#ifdef H5_HAVE_WIN32_API
+/* off_t exists on Windows, but is always a 32-bit long, even on 64-bit Windows,
+ * so we define HDoff_t to be __int64, which is the type of the st_size field
+ * of the _stati64 struct.
+ */
+#define HDoff_t __int64
+#endif /* H5_HAVE_WIN32_API */
+#ifndef H5_HAVE_WIN32_API
+/* These definitions differ in Windows and are defined in
+ * H5win32defs for that platform.
+ */
+#define HDoff_t off_t
+#endif
+
 /**
  * \ingroup FAPL
  *
@@ -6353,7 +6368,7 @@ H5_DLL herr_t H5Pset_dset_no_attrs_hint(hid_t dcpl_id, hbool_t minimize);
  * \since 1.0.0
  *
  */
-H5_DLL herr_t H5Pset_external(hid_t plist_id, const char *name, off_t offset, hsize_t size);
+H5_DLL herr_t H5Pset_external(hid_t plist_id, const char *name, HDoff_t offset, hsize_t size);
 /**
  * \ingroup DCPL
  *