Move `vPoller Helpers` from client applications to the `vPoller Worker`

* Will allow helper modules to be used by different clients as the
  helper modules are no longer client-side specific
Update the `vPoller Helpers` documentation
Cosmetics
Initial commit of the "vPoller loadable module for Zabbix"
Document how to build, install and configure the vPoller loadable module
for Zabbix
Mention to disable any Zabbix LLD rules if using `zabbix-vsphere-import` tool
Update usage information for vpoller-client and vpoller-cclient
Add support for vPoller helpers in the C client
When we cannot get a property for a vSphere object return '(null)' as a
string as other languages do not understand Python's None type
Typo fix
vpoller-cclient: Free any allocated memory by the ZeroMQ message and context
Fix a memory leak in vpoller-cclient
Change buffer size of the vPoller module for Zabbix to MAX_BUFFER_LEN bytes
Plug a memory leak in the vPoller loadable module for Zabbix

* valgrind(1) is happier now :)
Add logging to the vPoller Zabbix helper
Zabbix helper module fixes
The message we get from zmq_msg_data() is not null-terminated, so make
sure we terminate it before trying to release any memory or print it
If "result is None" then that means we got no response
zbx-helper: Discovery methods make use of the 'method' name, so set it
as attribute
Do not NULL-terminate the data we receive on the ZeroMQ socket

* We need to make sure that any data we pass from Python to C is already
  NULL-terminated