Add an optional task dumper thread

Creates an optional thread whose job is simply to poll for the existence of the file .dump in the current directory. When this file is found dumps of all the unskipped tasks and uncompleted entries in the queues is made, as is a dump of memory use, if --enable-memuse-reports was configured. Once the dump is complete the .dump file is removed and the application will just continue so must be killed normally, or can be dumped at a later time (this feature is aimed at checking deadlocks of the engine tasks, so not clear if that would be useful).

Uses configure flag --enable-dumper to enable, so not always available, so you need to catch the problem at least twice, but reliably works, unlike the signals version !857 (closed).