Fix zic for Windows.

The new coding of dolink() is dependent on link() returning an on-point
errno when it fails; but the quick-hack implementation of link() that
we'd put in for Windows didn't bother with setting errno.  Fix that.

Analysis and patch by Christian Ullrich.

src/timezone/zic.c

@@ -3485,7 +3485,10 @@ int
 link(const char *oldpath, const char *newpath)
 {
 	if (!CopyFile(oldpath, newpath, false))
+	{
+		_dosmaperr(GetLastError());
 		return -1;
+	}
 	return 0;
 }
 #endif