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/ex_docmd.c

@@ -6547,18 +6547,14 @@ void alist_slash_adjust(void)
 
 #endif
 
-/*
- * ":preserve".
- */
+/// ":preserve".
 static void ex_preserve(exarg_T *eap)
 {
   curbuf->b_flags |= BF_PRESERVED;
-  ml_preserve(curbuf, TRUE);
+  ml_preserve(curbuf, true, true);
 }
 
-/*
- * ":recover".
- */
+/// ":recover".
 static void ex_recover(exarg_T *eap)
 {
   /* Set recoverymode right away to avoid the ATTENTION prompt. */

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;

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;
   }
 }

src/nvim/memline.c

@@ -1593,7 +1593,7 @@ static int recov_file_names(char_u **names, char_u *path, int prepend_dot)
  * If 'check_char' is TRUE, stop syncing when character becomes available, but
  * always sync at least one block.
  */
-void ml_sync_all(int check_file, int check_char)
+void ml_sync_all(int check_file, int check_char, bool do_fsync)
 {
   FOR_ALL_BUFFERS(buf) {
     if (buf->b_ml.ml_mfp == NULL || buf->b_ml.ml_mfp->mf_fname == NULL)
@@ -1612,14 +1612,14 @@ void ml_sync_all(int check_file, int check_char)
       if (!os_fileinfo((char *)buf->b_ffname, &file_info)
           || file_info.stat.st_mtim.tv_sec != buf->b_mtime_read
           || os_fileinfo_size(&file_info) != buf->b_orig_size) {
-        ml_preserve(buf, FALSE);
+        ml_preserve(buf, false, do_fsync);
         did_check_timestamps = FALSE;
         need_check_timestamps = TRUE;           /* give message later */
       }
     }
     if (buf->b_ml.ml_mfp->mf_dirty) {
       (void)mf_sync(buf->b_ml.ml_mfp, (check_char ? MFS_STOP : 0)
-          | (bufIsChanged(buf) ? MFS_FLUSH : 0));
+                    | (do_fsync && bufIsChanged(buf) ? MFS_FLUSH : 0));
       if (check_char && os_char_avail())        /* character available now */
         break;
     }
@@ -1636,7 +1636,7 @@ void ml_sync_all(int check_file, int check_char)
  *
  * when message is TRUE the success of preserving is reported
  */
-void ml_preserve(buf_T *buf, int message)
+void ml_preserve(buf_T *buf, int message, bool do_fsync)
 {
   bhdr_T      *hp;
   linenr_T lnum;
@@ -1656,7 +1656,7 @@ void ml_preserve(buf_T *buf, int message)
 
   ml_flush_line(buf);                               /* flush buffered line */
   (void)ml_find_line(buf, (linenr_T)0, ML_FLUSH);   /* flush locked block */
-  status = mf_sync(mfp, MFS_ALL | MFS_FLUSH);
+  status = mf_sync(mfp, MFS_ALL | (do_fsync ? MFS_FLUSH : 0));
 
   /* stack is invalid after mf_sync(.., MFS_ALL) */
   buf->b_ml.ml_stack_top = 0;
@@ -1686,7 +1686,7 @@ void ml_preserve(buf_T *buf, int message)
     }
     (void)ml_find_line(buf, (linenr_T)0, ML_FLUSH);     /* flush locked block */
     /* sync the updated pointer blocks */
-    if (mf_sync(mfp, MFS_ALL | MFS_FLUSH) == FAIL)
+    if (mf_sync(mfp, MFS_ALL | (do_fsync ? MFS_FLUSH : 0)) == FAIL)
       status = FAIL;
     buf->b_ml.ml_stack_top = 0;             /* stack is invalid now */
   }

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;
     }
   }

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