upscmd/upsrw: make the "OK" without "TRACKING" warning visible…

[jimklimov]

…introduce NUT_QUIET_OK_NOTRACKING envvar to hide it though

Closes: #3383

Example:

:; upscmd  -u user -p pass eco650@localhost beeper.mute
OK
WARNING: 'OK' only means the NUT data server accepted
the request as valid, but as we did not wait for result,
we do not know if it was handled in fact.
You can export NUT_QUIET_OK_NOTRACKING=true to hide this message,
or use -w [-t SEC] option(s) to track the actual outcome.

:; NUT_QUIET_OK_NOTRACKING=true upscmd  -u user -p pass eco650@localhost beeper.mute
OK

Note that both the warning and the value returned by the server and reported by upscmd/upsrw (e.g. OK or an error code) are currently printed to stderr. Hopefully nobody relies on parsing that stream, as there may be various other outputs there anyway. Maybe the value from server should be reported on stdout, but that would be a separate issue.