changing how we specify non-standard settings

- making it a characteristic of each session (instead of a
  global flag that is enabled or not)
- updated the core code to remove the check for extensions
  (putting that responsibility on the code that sets up each
  session at the moment)
- modified the bfd executable to support this change
update bfdd handling of extended functionality

- remove bfdExtensions.[ch] (no longer needed)
- update format of config file to put extended (non-standard)
  functionality in its own section
- make extended functionality configured per-session (instead
  of globally)
make config file optional