Fix a minor regression in upsclient output

NSS support has introduced a minor regression in upsclient output.
Clients such as upsc, upscmd and upsrw were particularly affected.
This patch restores a default behavior similar to prior versions.
However, "-v" option remains to be implemented.

Address the first task of Github issue #30
Complete upsclient commands usage note

Add a note for scripting usage, for upsc, upscmd and upsrw, to state the
obvious: only consider the output from stdout for data requested. stderr
may contain error messages, which can disrupt your script execution.

Address the second task and closes Github issue #30