Add edit-content-type helper and warning for decrypted attachments. (closes #3728)

Add edit-content-type helper and warning for decrypted attachments. (closes #3728)

Regenerating the actx index will overwrite any changes made to a
decrypted attachment.  Change the mutt_edit_content_type() function to
return 1 when a structural change is made.  Add a warning message when
that is the case and a decrypted message was edited, so the user is
not surprised.

Note: mutt_edit_content_type() appeared to regenerate multipart
subparts every time, leading to a memory leak.  I believe this was an
oversite, and it should have regenerated only when there were no
subparts, so have "fixed" this.