autosave: add option for permissions on autosave JSON files

[klauer]

State

For the autosave IOC helper, NamedTemporaryFile is used to write the autosave state in JSON format, which is then moved/renamed to its date/timestamped filename.

Issue

NamedTemporaryFile does not respect the user's default umask, as temporary files are generally intended to only be accessible by the owner.

This means that for IOCs run as services (or under procserv) with a special "IOC user" may write files that are not readable by others.

Fix

• Allow the IOC author to configure the autosave file's permissions
• Set permissions when writing out autosave files