Zabbix

...G...... [ZBXNEXT-3687] Splitted logic for zbx_shm_* functions

The previous zbx_shmget() function, was taking care of allocating and
eventually substituting a shared memory segment. Now we splitted the
logic into two small separated functions.
zbx_shm_create() will take only a size and give back an ID for a shared
memory segment, while zbx_shm_destroy() will mark the shared
memory identified by the shmid parameter for destruction. To note that
zbx_shm_destroy() will not take care of detaching the shared memory
segment from the process, that's responsibility of the caller.
Also, give the functions names more consistent with the general style
of the zabbix code conventions.

Suggested by @wiper