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