[3.12] gh-115886: Handle embedded null characters in shared memory name (GH-115887) (GH-115906)

shm_open() and shm_unlink() now check for embedded null characters in
the name and raise an error instead of silently truncating it.
(cherry picked from commit 79811ededd160b6e8bcfbe4b0f9d5b4589280f19)