rndctl: make -l's "Flags" field reflect current reality

don't print "legacy options" RND_FLAG_ESTIMATE_TIME and
"RND_FLAG_ESTIMATE_VALUE"

only print "estimate" if we have actually counted any bits from
something, since it's no longer really possible to "enable estimation".

ideally, there should also be a "samples" field so it's clear
collected bits are not being counted.

(nia)