Break libcheri's 'sandbox_stat.h' into two different files: an identically
named file in libprocstat, which defines the ABI between sandboxed programs
and procstat(1), and 'libcheri_stat.h' which is an internal header inside
of libcheri.  This fixes the build using WITHOUT_CHERI=yes as libprocstat
and procstat can now find sandbox_stat.h.  We hope to use this with
Capsicum as well, so it should not be considered CHERI-specific.