Add "brief" option to rxgen

Add "brief" option to rxgen

Add a new -b option to rxgen that turns on "brief" output. This makes a
number of changes to the data definitions produced by rxgen so they can
be more easily used by the calling code.

The changes are:
   *) Use the new struct rx_opaque structure for all opaque data
      definitions, rather than defining each as a unique structure.
      This permits moving opaque data between rxgen structures to be
      performed by simple assignment.
   *) Use anonymous structures for internal definitions. Currently
      rxgen also uses the field name as the structure name, which
      prevents the use of a field name more than once within a
      source file.
   *) Don't embed the structure name within the names of the elements
      within the structure. This significantly reduces the length of
      assignment code, and makes for more readable callers.

Change-Id: I8cad7e6051f12238a77cf006b0854fb38b54f61a
Reviewed-on: http://gerrit.openafs.org/2585
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>