fixup! A pessimized implementation of thread-safe calls
Exported on- and off-stack call paths to Rust

So that Rust code gets to use stack closures
with the fast on-stack call path.
fixup! A pessimized implementation of thread-safe calls
Added a test for off-stack calls

The test either crashes or hangs up.
Some support for debug builds

Now you can do `make BUILD_OPTIONS=debug`.
Debug printouts for refcounting
Disable gdbus/gvfs usage when running tests

The gvfsdbus backend adds a lot of runtime state and activity
highly dependent on local configuration. Even if it may reveal
problems in our event loop code, the added noise and lack of
predictability is unwelcome for testing and the debug tools.