Change (again), and simplify, the file model inference algorithm

[DennisHeimbigner]

• For URL paths, the new approach essentially centralizes all information
  in the URL into the "#mode=" fragment key and uses that value
  to determine the dispatcher for (most) URLs.

• The new approach has the following steps:

  1. canonicalize the path if it is a URL.
  2. use the mode= fragment key to determine the dispatcher
  3. if dispatcher still not determined, then use the mode flags
     argument to nc_open/nc_create to determine the dispatcher.
  4. if the path points to something readable, attempt to read the
     magic number at the front, and use that to determine the dispatcher.
     this case may override all previous cases.

• Misc changes.

  1. Update documentation
  2. Moved some unit tests from libdispatch to unit_test directory.
  3. Fixed use of wrong #ifdef macro in test_filter_reg.c
     [I think this may fix an previously reported esupport query].