Fix unwanted IMAP connections on startup caused by processing rules.

Fix unwanted IMAP connections on startup caused by processing rules.

The culprit was a combination of folder_item_prefs_clear()
setting item->enable_processing to TRUE even though default
value should be FALSE, and initial_processing() not skipping
over root folders, which do not have this variable later set
to correct default value in xml_to_folder_item().

Closes bug #3993: Claws Mail connects to IMAP server when it should not