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/misc1.c

@@ -2643,7 +2643,7 @@ void preserve_exit(void)
     if (buf->b_ml.ml_mfp != NULL && buf->b_ml.ml_mfp->mf_fname != NULL) {
       mch_errmsg((uint8_t *)"Vim: preserving files...\n");
       ui_flush();
-      ml_sync_all(false, false);    // preserve all swap files
+      ml_sync_all(false, false, true);  // preserve all swap files
       break;
     }
   }