KOSync: Clarify settings, plus refactor & fixes to make "auto-sync" more reliable (#10605)

Fix: #10539, and for context #6489, #6733, #6534

Reorganize and reword most of the settings to make it clear what actually ties into auto sync, and what doesn't. (Specifically, what happens when a pull attempts to sync forward or backward has nothing to do with auto sync, it applies in all cases; while the periodic sync *does* require auto sync).

The main point of contention, though, is that auto sync will now *always* attempt to setup network connectivity (i.e., on resume/suspend/close). Periodic sync will *not* though (the intent being that, if you use periodic sync, you're relying on the activity check to actually keep wifi on at all times)).

Since this may lead to a large amount of nagging about wifi toggles on devices w/ NetworkManager support, it is now *disabled* by default on those devices. (And given that it wouldn't have worked because of the lack of connectivity, that doesn't really make any practical difference ;p).

Additionally, given the fact that there's no way to make this behavior viable if the "before wifi" action is left at its default of "prompt", this feature now *requires* that to be set to "turn_on" (on devices where it can, of course); attempting to toggle it on will warn about that if necessary.
This change is retroactive (OTM).

Includes an assortment of fixes and cleanups, including migrating to the new LuaSettings API, which is why there's no longer a smattering of superfluous flushes.

frontend/apps/reader/modules/readerwikipedia.lua

@@ -244,7 +244,7 @@ function ReaderWikipedia:addToMainMenu(menu_items)
                     -- home_dir/Wikipedia/
                     if not G_reader_settings:readSetting("wikipedia_save_dir") then
                         local home_dir = G_reader_settings:readSetting("home_dir")
-                        if not home_dir or not lfs.attributes(home_dir, "mode") == "directory" then
+                        if not home_dir or lfs.attributes(home_dir, "mode") ~= "directory" then
                             home_dir = require("apps/filemanager/filemanagerutil").getDefaultDir()
                         end
                         home_dir = home_dir:gsub("^(.-)/*$", "%1") -- remove trailing slash