Specify directory for temporary captures

Add a capture option --temp-dir to wireshark/tshark/dumpcap that specifies the directory in which temporary capture files are to be written. If not specified, uses the system default behaviour.

Add a GUI element in the capture options dialog for specifying this directory.

I found that this couldn't be effective as a user preference; it had to be a capture option in order to be successfully propagated to dumpcap child processes.

A lot of internal function signatures were changed so that the directory location could ultimately be passed to create_tempfile().

Closes #16432 (closed)