Refactor MIME support,

Refactor MIME support,

- to make sense,
- to be more robust,
- to better reflect the terminology of the RFCs,
- to simplify code that uses MIME bodies,
- to move all generic MIME code into imail-mime.scm, and
- to fix a number of small bugs.

The documentation will need to be updated, since some user-visible
commands are now named with the word `body' rather than `entity'.

Undoubtedly, refactoring added many small bugs, too, still to be
weeded out.