mutt: Add confirm patches and mua flag

This adds two additional patches I find useful; confirm-crypt-hook (adds the
option to skip the "Use KeyID" prompt) and confirm-attachment (prompts you if
you forgot to add attachments).

Note I mirrored the latter on gist as using original mailing list attachment
failed in brew (perhaps due to querystrings in the URL?).

I also added the option to disable building with IMAP/SMTP etc. support. This
can be useful if mutt is used as a traditional MUA in combination with other
tools (offlineimap, msmtp etc.).