Remove dead pread and pwrite replacement code.

pread() and pwrite() are in SUSv2, and all targeted Unix systems have
them.

Previously, we defined pg_pread and pg_pwrite to emulate these function
with lseek() on old Unixen.  The names with a pg_ prefix were a reminder
of a portability hazard: they might change the current file position.
That hazard is gone, so we can drop the prefixes.

Since the remaining replacement code is Windows-only, move it into
src/port/win32p{read,write}.c, and move the declarations into
src/include/port/win32_port.h.

No need for vestigial HAVE_PREAD, HAVE_PWRITE macros as they were only
used for declarations in port.h which have now moved into win32_port.h.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Greg Stark <stark@mit.edu>
Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CA+hUKGJ3LHeP9w5Fgzdr4G8AnEtJ=z=p6hGDEm4qYGEUX5B6fQ@mail.gmail.com

src/port/win32pread.c

@@ -0,0 +1,44 @@
+/*-------------------------------------------------------------------------
+ *
+ * win32pread.c
+ *	  Implementation of pread(2) for Windows.
+ *
+ * Portions Copyright (c) 1996-2022, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *	  src/port/win32pread.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+
+#include "postgres.h"
+
+#include <windows.h>
+
+ssize_t
+pread(int fd, void *buf, size_t size, off_t offset)
+{
+	OVERLAPPED	overlapped = {0};
+	HANDLE		handle;
+	DWORD		result;
+
+	handle = (HANDLE) _get_osfhandle(fd);
+	if (handle == INVALID_HANDLE_VALUE)
+	{
+		errno = EBADF;
+		return -1;
+	}
+
+	overlapped.Offset = offset;
+	if (!ReadFile(handle, buf, size, &result, &overlapped))
+	{
+		if (GetLastError() == ERROR_HANDLE_EOF)
+			return 0;
+
+		_dosmaperr(GetLastError());
+		return -1;
+	}
+
+	return result;
+}