In plain text mails, format any signature as fixed spaced

It may not be clear what kind of formatting a user would expect for plain text emails, but at least for the signature part - if any - a fixed font seems to make a lot of sense. Using "pre-wrap", so overly long lines will still wrap.

Note that I do not actually think this code belongs into the model. I'd rather expect any kind of formatting to happen in the viewer code, exclusively (and, prospectively, be controlled by user options). I'm following the existing pattern, though, in order to not risk breaking any existing usage.