Mostly, I kept the existing source skeletons in cap.c and
cptr_cache.c (they've just moved to src/common), but all the
user/kernel include differences are nicely factored out.  You
might think my organization is a bit schizophrenic (a flattened
include/ dir, and a hierarchical src/ dir), but I do that because
I don't particularly care for libraries that install headers in
$PREFIX/include/libfoo/{subdir1,subdir2,...}.  The idea with the
src/ dir is that common cap/cptr logic goes in src/common, and
any "platform" specialization (user lib vs kernel mod) goes in
src/user or src/kernel .  The libcap.h and libcap_internal.h
headers define some common types, macros, and functions, and expect
the platform headers and source files to specialize them.

I added a very basic notion of capability object types and tied it
to revocation and deletion in the same way the original library did.

For userspace, since I didn't have atomic ops, I just did basica...