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/os/signal.c

@@ -145,7 +145,7 @@ static void on_signal(SignalWatcher *handle, int signum, void *data)
     case SIGPWR:
       // Signal of a power failure(eg batteries low), flush the swap files to
       // be safe
-      ml_sync_all(false, false);
+      ml_sync_all(false, false, true);
       break;
 #endif
 #ifdef SIGPIPE