Ignore subkeys in apt-key's output

Ignore subkeys in apt-key's output

The output of `apt-key adv --list-keys --with-colons --fingerprint --fixed-list-mode` is always a `pub` line followed by one of more `fpr` lines (for subkeys, if any). This can cause the wrong pairs of `pub` and `fpr` lines to be hashed, resulting in nonsense.

The ordering `pub`, `fpr`, `pub`, `fpr` is guaranteed by `apt-key`.

Therefore, this commit clears `fpr_line` when a `pub` line is encountered, to reset the grouping. The current code ignores subkeys anyway, so this is no more bad.