inboxwritable: mime_from_path: reuse in more places

inboxwritable: mime_from_path: reuse in more places

There's nothing Maildir-specific about the function, so
`maildir_path_load' was a bad name.  So give it a more
appropriate name and use it in our tests.

This save ourselves some code and inconsistency by reusing an
existing internal library routine in more places.  We can drop
the "From_" line in some of our (formerly) mbox sample files.