Basic fixes to calibre-sync (#3558)

* Properly create intermediate directories when receiving books from Calibre.

This fixes an issue where you can't receive books except into directories that
already exist on the Kobo, which, in particular, causes problems when your
configuration in Calibre is something like "put books in $Author/$Title.epub"
and you haven't previously synced any books by that author.

* Wake up periodically to process ZMQs if any are registered.

This fixes an issue where if there are any timed events (such as the suspend
timer) in the queue, ZMQ events may not get processed until the timed event
fires, which is a problem when (for example) the suspend timer goes off in
an hour and you have something trying to send a book to the kobo over wifi
*right now*.

With this change, the event loop will wake up every 50ms to check for ZMQ
events and process them if necessary. If there are no ZMQs registered (which
is typical), it uses the original behaviour -- so this won't affect battery
life under normal usage.

frontend/util.lua

@@ -304,6 +304,32 @@ function util.isEmptyDir(path)
     return true
 end
 
+--- Checks if the given path exists. Doesn't care if it's a file or directory.
+---- @string path
+---- @treturn bool
+function util.pathExists(path)
+    local lfs = require("libs/libkoreader-lfs")
+    return lfs.attributes(path, "mode") ~= nil
+end
+
+--- As `mkdir -p`.
+--- Unlike lfs.mkdir(), does not error if the directory already exists, and
+--- creates intermediate directories as needed.
+---- @string path the directory to create
+---- @treturn bool true on success; nil, err_message on error
+function util.makePath(path)
+    path = path:gsub("/+$", "")
+    if util.pathExists(path) then return true end
+
+    local success, err = util.makePath((util.splitFilePathName(path)))
+    if not success then
+        return nil, err.." (creating "..path..")"
+    end
+
+    local lfs = require("libs/libkoreader-lfs")
+    return lfs.mkdir(path)
+end
+
 --- Replaces characters that are invalid filenames.
 --
 -- Replaces the characters <code>\/:*?"<>|</code> with an <code>_</code>.
@@ -326,7 +352,9 @@ function util.replaceSlashChar(str)
     end
 end
 
---- Splits a file into its path and name
+--- Splits a file into its directory path and file name.
+--- If the given path has a trailing /, returns the entire path as the directory
+--- path and "" as the file name.
 ---- @string file
 ---- @treturn string path, filename
 function util.splitFilePathName(file)