Unlink this program from the shared memory segment specified in
shm_id. This is the return value from the function that created or
linked to the segment. Avoid creating a second open link to the same
shared memory segment in the same program. If this is done, it is
necessary to unlink twice, once with each shm_id.
When all programs, including the one that created the segment, have
unlinked it or exited, the shared memory is released (except when created
with the SHM_PERSIST option).