DTrace depends on an obsolete debugfs mount point

[nickalcock]

The old /sys/kernel/debug/tracing mount point for the tracefs has been obsolete for some time (not sure how long, sometime between kernel 4.3 and kernel 6.1). The new location is /sys/kernel/tracing, but some installations don't mount it yet so debugfs automounts the tracefs under the old location in that case: it even seems to be possible to get both directories, or to have both directories but one of them is empty. This means we have to cope with a debugfs that might be on /sys/kernel/debug/tracing or /sys/kernel/tracing on different systems or even on the same system at different times.

We should dynamically adapt to such things.

[nickalcock]

Implemented a fix, under test now.

[nickalcock]

Pushed for experimentation to the nix/movable-tracefs branch (not yet reviewed, may do anything!)

[nickalcock]

Revised thing pushed -- almost totally different, courtesy of a really good review from Kris. Still seems to work plus is much better :)