shm_open2(2): completely unbreak
kern_shm_open2(), since conception, completely fails to pass the mode along
to kern_shm_open(). This breaks most uses of it.
Add tests alongside this that actually check the mode of the returned
files.
PR: 240934 [pulseaudio breakage]
Reported by: ler, Andrew Gierth [postgres breakage]
Diagnosed by: Andrew Gierth (great catch)
Tested by: ler, tmunro
Pointy hat to: kevans