IO: let 'fsync' option control more cases

Vim has the 'swapsync' option which we removed in 62d137ce09.
Instead let 'fsync' control swapfile-fsync.

These cases ALWAYS force fsync (ignoring 'fsync' option):
- Idle (CursorHold).
- Exit caused by deadly signal.
- SIGPWR signal.
- Explicit :preserve command.

src/nvim/fileio.c

@@ -3068,7 +3068,7 @@ nobackup:
    */
   if (reset_changed && !newfile && overwriting
       && !(exiting && backup != NULL)) {
-    ml_preserve(buf, FALSE);
+    ml_preserve(buf, false, !!p_fs);
     if (got_int) {
       SET_ERRMSG(_(e_interr));
       goto restore_backup;