mhparam: Added more items for direct query and -debug listing.
Most new values are not included in the -all listing because they
are not part of the profile or context but are mainly internal
configuration values.
The -debug listing has now section headings (pipe through
`grep -v ^#' to remove them).
The motivation for this change was querying the default inbox
name. If `Inbox' wasn't set in the profile, `mhparam inbox' had
failed. Now it prints `+inbox' (if the defaultfolder has not been
changed on your installation).