Add some notes on what has been done, and what remains


(pgoyette)

DONE
----
1. Removed linking of the kernel compat object library into all kernels;
   using the .o library caused some build breakage, and resulted in the
   bulk of the compat code being included in every kernel, but without
   any module linkage.  In turn, this caused failure when loading some
   modules due to symbols already being defined in the kernel.

2. Reverted some intentional breakage for loading the sysv_ipc module;
   the breakage was introduced as the fix for the above-mentioned build
   breakage.  

3. Split the sysv_ipc compat routines into their own compat_sysv module.

4. Resolved some inter-module dependencies.

5. Extracted some net/if.c compat routines into the compat module, and
   replaced the originals with indirect (vectored) function calls.


TODO
----
1. Audit the entire code base for any remaining embedded #ifdef's for
   COMPAT_xx.  When found, move the actual compat code into the compat
   hierarchy and replace originals with indirect (vectored) calls.

   (There are several instances of this in the net routing table code.)

2. Implement a mechanism for modules to include "aliases".  This would
   allow a single module file to contain multiple module table entries
   (for example, a single file for the compat module could include
   compat_80, compat_70, compat_60, ...).  Then, other modules could
   depend on individual names.  For example, module compat_xyz could
   depend on compat_70, and the dependency would be met by either the
   "everything" compat module (with all of its aliases) or by a 7.0-
   specific module.

   Note that in addition to updating the module code, this would also
   require changes to syscalls.master files to change the names of the
   modules associated with module-provided syscalls.

3. In addition to the single, all-encompassing compat module, provide a
   per-version compat_xx module, each with dependencies on all higher
   versions.  (Perhaps we would also do this for the sysv ipc module,
   although the gains are a lot smaller.)