Refactor the internal_feature_available to return the result for just
one group, so it is easier to know on the library side what was the
actual error, and which group refers to; drop internal_available, as no
more needed after this.
On the library side, implement in available and feature_available the
real logic to iterate through the requested group, and error out or
return whether the groups are available. This also introduces caching
for the features, so each needs to be queried just once in each
appliance run.
The result of this is that there should be much less communication with
the daemon to know about available features; the downside is that
queries for more than one group at once, not already cached, will be
cause more queries to the daemon.
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