Allow control over background task cancellation
Control cancellation for delete, copy and move
Forbid cancellation for background operations

 - delete
 - copy
 - move
Move fileops.c:cp_file below its last invocation
Move fileops.c:mv_file below its last invocation
Fix declarations for fileops.c:(cp|mv)_file()
Merge branch 'fix-bg-file-tasks'

Normal work of backgrounded operations was a bit broken by introducing
cancellation.  Explicitly forbid cancellation for background operations.