util,turnip: Add file notifier utility and runtime TU_DEBUG toggling support

Adds a utility to watch files for changes or deletions using inotify, based on RADV's VRS config notifier. On top of which runtime toggling of a subset of TU_DEBUG flags is implemented, via specifying a file (with TU_DEBUG_FILE) to read options from and update them immediately.

Note: The RADV notifier implementation could be migrated to the new utility, but I have no way to test those changes so I've not done that myself.