Fix default for custom tracing

[macpie]

By default, file named tracing.cfg should be created where executable is started. This can be changed via config:

[custom_tracing]
tracing_cfg_file = "/tmp/tracing.cfg"

Notes:

• File can be created after.
• Deletion of the file will remove custom filter (tested using rm file but for some reason when doing in editor it does something weird)
• Empty the file will remove the custom filter.