nut plugin: Multi-threaded ups polling with connect timeout.

nut plugin: Multi-threaded ups polling with connect timeout.

Changes:

* Implemented use of 'plugin_register_complex_read', so querying will be multi-threaded;
* Added 'ConnectTimeout' option, implemented with use of 'upscli_tryconnect()' added in nut-2.6.2;
* The `upscli_cleanup()` should be called only at Collectd shutdown, excessive calls was removed;
* Added `upscli_init()` call if `ForceSSL` enabled.
* Added a check for duplicated values in `UPS` option.

Closes: #2344