catalyst/targets/generic_stage_target.py: mount /dev/shm on linux

catalyst/targets/generic_stage_target.py: mount /dev/shm on linux

Add shm targets defaults. Anthony G. Basile <blueness@gentoo.org>
Some build systems require /dev/shm to be mounted, like python's
build system.  We make sure that on Linux systems, /dev/shm is
mounted in the stage chroots.  See bug #496328.

Douglas Freed <dwfreed@mtu.edu> :
Mount /dev/shm in the chroot with the right options
Bind mounting /dev/shm into the chroot isn't a good idea, as there may
be collisions and result in weird side effects.  Instead, we can just
mount a new tmpfs there, with the right options to ensure security.

(Forward ported to pending branch from 2.X Brian Dolbec)
Conflicts:
	catalyst/targets/generic_stage_target.py