Improve handling of message/external-body

[Katsumi Yamaoka <yamaoka@jpl.org>]

Hi,

I found there are several problems concerned with externalized
MIME parts that I made performing the `C-o' command on ordinary
MIME parts.

1. Each time I type the RET key, displaying of inlinable parts,
   e.g. text/plain parts, isn't toggled.

2. Displaying of inlined images cannot be turned off using the
   RET key.

3. Several MIME part commands, e.g. `i',`o', `|', etc. pass only
   headers of the externalized parts to the destinations (e.g.
   buffer, file, command).

4. When saving a not inlinable part to a file, the default file
   name isn't offered.

Fixed in the trunk and the v5-10 branch.