I have a few remarks:

 - in prop_scn.c you need to rename "struct frame" - it collides with
   kernel struct frame for most archs.

 - I would suggest to do the "detection" of codecs when internazlizing
   completely differnt, by making it completely internal to the codec.
   prop_codec_guess() should not know anything about the externalized
   data, do not skip whitspace before calling it and pass it a small
   buffer of data (with length) - and then call a codec provided
   probe function.

 - IMHO an application should have a mean to select the codec when 
   externalizing to file at runtime, if different codecs are compiled
   in.

 - The make framework should offer different sets of codecs for the
   kernel version. Actually I will suggest that only the upcoming
   "lbd" (local binary data) codec should be available inside the
   kernel.

 - The codec selection for the ioctl handlers should be specialized
   (i.e. if the previous point gets implemented, only "lbd" should
   be used to send ioctl data from userland to kernel).

Martin