blk: use choose_fd for all filehandle references

blk: use choose_fd for all filehandle references

This is a leading part of the changes to implement bluestore on
FreeBSD. Without this invalid indexing in the descriptor arrays
will occur.

Creates a new enum: `blk_access_mode_t`
to describe `BUFFERED` and `DIRECT `mode access
with `choose_fd()` to get the correct file for the typed access
and adds
a pretty-printer
boolean convertor `blk_access_mode_t::buffermode(bool)`

This PR is a redo for PR #37258, since that one was lost
in rebasing errors. But the review notes are still there.

Signed-off-by: Willem Jan Withagen <wjw@digiware.nl>