Allow to configure ALLOW_NO_DEVICE (nee NUT_NOCONF_ALLOWED) option via upsd.conf

[jimklimov]

Follow-up from #102

Tested to support different ways of spelling the boolean setting in upsd.conf and in envvar as supported (and with higher priority) per original PR.

[boricj]

Code looks good, but I'll bike-shed the name of the configuration key. This is the only configuration key with underscores and the only prefixed by NUT_.

I won't demand a change, but do consider making things consistent.

[jimklimov]

I agree, that concerned me too. The name originally came to be like the other envvars recognized by the code (and "namespaced" into NUT_). I suppose if Arno has no comments on this soon, we can merge this as is for 42ity and later solve "properly" with equivalent PR in upstream NUT.

For 42ity use-case, currently this setting is easier to propagate via envvars of nut.conf as (and/or because) it is already done in earlier PR. So a renaming change coming back from upstream (or a next commit here) to the upsd.conf option name will not hurt anything :)

[aquette]

@jimklimov ack, letting pass for now. But please remember me / other (ticket) later that we have a tech debt to fix. Will try to PoC... later when I got time and WR20 is out and cooling stage 1 is delivered.
waiting for the green light, but approving for you to merge and not lagging after me ;)