add commands for working with (background) scanner

[icewind1991]

• occ files_antvirus:status [-v] to get info about files in the scan queue
• occ files_antivirus:background-scan [-v] [-m MAX] to manually trigger the background scan
• occ files_antivirus:scan <path> to manually scan a single file
• occ files_antivirus:mark <path> <scanned|unscanned> to mark a file as scanned/unscanned

[icewind1991]

Added an extra command