Introduce xcb_aux_alloc_shm() to create anonymous files in RAM
... which then can be mapped with mmap(). It is intended to be used in conjunction with xcb_shm_attach_fd() to access the created shared memory from a client and the X server.
The implementation is based on the code of shm_tmpfile() from X.Org Server.
Signed-off-by: Alexander Volkov <a.volkov at rusbitech.ru>