Zabbix

...G...... [ZBXNEXT-3687] Don't use keys for allocating shared memory anymore

The zbx_ftok() function was completely removed from the source code. The
function was used only inside the agent code and the zbxnix library.

The zbx_shmget() function was reworked for not using a key but for using
only an id for checking if a shared memory segment exist. By the logic of
the allocating code, we can skip the check with the key, because we always
check for errors when the first instances of the shared memory segments
are created. The logic behind the shared memory management in the
agent didn't changed.