ignore unsupported key formats in apt-key

gpg2 generates keyboxes by default and users end up putting either those
or armored files into the trusted.gpg.d directory which apt tools
neither expect nor can really work with without fortifying backward
compatibility (at least under the ".gpg" extension).

A (short) discussion about how to deal with keyboxes happened in
https://lists.debian.org/deity/2017/07/msg00083.html
As the last message in that thread is this changeset lets go ahead
with it and see how it turns out.

The idea is here simply that we check the first octal of a gpg file to
have one of three accepted values. Testing on my machines has always
produced just one of these, but running into those values on invalid
files is reasonabily unlikely to not worry too much.

Closes: #876508

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