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/getchar.c

@@ -1347,14 +1347,17 @@ void before_blocking(void)
  * All the changed memfiles are synced if c == 0 or when the number of typed
  * characters reaches 'updatecount' and 'updatecount' is non-zero.
  */
-void updatescript(int c)
+static void updatescript(int c)
 {
   static int count = 0;
 
-  if (c && scriptout)
+  if (c && scriptout) {
     putc(c, scriptout);
-  if (c == 0 || (p_uc > 0 && ++count >= p_uc)) {
-    ml_sync_all(c == 0, TRUE);
+  }
+  bool idle = (c == 0);
+  if (idle || (p_uc > 0 && ++count >= p_uc)) {
+    ml_sync_all(idle, true,
+                (!!p_fs || idle));  // Always fsync at idle (CursorHold).
     count = 0;
   }
 }