Don't define the in-memory layout of CHERI capabilities in struct chericap
unless CHERICAP_INTERNAL is defined.

In various bits of kernel and userspace where the CHERI-aware compiler is
not used, extract capability fields using CGet* instructions rather than
direct memory inspection.  In a couple of places associated with the CHERI
compositor, define CHERICAP_INTERNAL, but we'll want to fix them as well
in the future.