Introduce a new GFileMonitor backend: libinotify-kqueue

libinotify-kqueue is a library that implements inotify interface in terms of kqueue/kevent API available on Mac OS and *BSD systems. The original kqueue backend seems to be a predecessor version of the code that is currently present in libinotify-kqueue. Under the hood the library implements a sophisticated filesystem changes detection algorithm that is derived from the glib backend code.

Updating the native glib kqueue backend requires substantial work, because code bases have diverged greatly. Another approach is taken, instead. libinotify-kqueue can serve as a drop-in replacement for Linux inotify API, thus allowing to reuse the inotify backend code. The compatibility, however, comes at cost, since the library has to emulate the inotify descriptor via an unix domain socket. This means that delivering an event involves copying the data into the kernel and then pulling it back.

The recent libinotify-kqueue release adds a new mode of operation called "direct". In this mode the socket pipe is replaced with another kqueue that is used to deliver events via a kevent(EVFILT_USER) call. Employing the direct mode requires minor changes to the client code compared to using plain inotify API, but in return it allows for reusing libinotify's algorithms without a performance penalty. Luckily, all required changes are consolidated in one file called inotify-kernel.c

This puts us in the best of possible worlds. On one hand we share a lot of code with glib inotify backend, which is far more thoroughly tested and widely used. On the other we support a range of non-Linux systems and consolidate the business logic in one library. I plan to do the same trick for QFileSystemWatcher which will give us the same behaviour between Gtk and Qt applications.

The glib test suite passes for both old kqueue backend and new libinotify-kqueue one. However, the AppStream FileMonitor tests are failing with the old backend, but pass with the new one, so this is still an observable improvement.

Relevant libinotify-kqueue PR: https://github.com/libinotify-kqueue/libinotify-kqueue/pull/19