Add support for delete-queue

[chros73]

Add support for .delqueue directory and refactor corresponding code.

The purpose of delete-queue is to be able to specify torrents (in the form of symlinks pointing to their metafile in this .delqueue directory) that will be deleted at first by a future queue script.

1. Add new boolean flag to store this: d.custom=delqueue
2. Create the following methods:

• d.add_to_delqueue adds the selected torrent to delqueue only if it has the unsafe_data flag
• d.remove_from_delqueue removes the selected torrent from delqueue
• d.fix_delqueue_flag fixes delqueue flag, apply only if unsafe_data flag and symlink in delqueue dir exists, remove it otherwise .
  Run this once per day: it's useful when a script creates these symlinks and we want to update this property.

1. Include delqueue support when data deleting happens.
2. This d.custom=delqueue flag (along with d.custom=unsafe_data flag) will be visible in the patched rtorrent-ps: Add 2 more columns on collapsed view rtorrent-ps-ch#3
3. Modify existing d.move_to manual moving method to support this new flag and reorder its parameters:

• it receives 4 arguments: dirname,specialgroup,unsafe_data,delqueue
• d.move_to=rotating,1,1, or d.move_to=rotating,1,1,1
• d.move_to=fullseed,1,,
• d.move_to=unsafe,,1, or d.move_to=unsafe,,1,1
• d.move_to=others,,,