Cleanup: Move `create_posix_shm()` to common/shm.c

Cleanup: Move `create_posix_shm()` to common/shm.c

* This function will be used for trigger error accounting.

* This function is renamed to `shm_create_anonymous()` and now takes one
  parameter for the owner of the shared memory area. Again this is code
  reuse.

* Remove erroneous comment about the `name` parameter for the
  `shm_open()` function. It's the absence (and not its presence) of the
  '/' at the beginning of the name that makes the behaviour implementation
  defined.

* Move the bin/lttng-sessiond/shm.c to common/shm.c so that all shm
  related functions are in one location.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I690b38801ffb4772e7c5069c4cccf470b0671f63