support non-autocrypt keys with muacrypt

[hpk42]

The current "mutt2" branch supports better integration with mutt (soon to be documented and releases) and in practise it turns out that muacrypt needs to offer a way to import/flag a key "as an autocrypt key". I think importing a key should:

• be virtually the same as if a mail arrived with that key
• by default set prefer-encrypt=nopreference with an option to force it to be "mutual".
• allow to force an e-mail address to be associated with the key (by default all UIDs would be considered as e-mail addresses)

[hpk42]

you can do "muacrypt import-public-key" with 0.9.0