capsicum: fix #ifdefs for other builds
capsicum: remove capability LSM hooks

The LSM hooks for .file_lookup and .file_install would
not be straightforwardly useable by the real LSM, so
don't bother with a hook and go straight to the
Capsicum code.
seccomp: Add tgid and tid into seccomp_data

Add the current thread and thread group IDs into the data
available for seccomp-bpf programs to work on.  This allows
installation of filters that police syscalls based on thread
or process ID, e.g. tgkill(2)/kill(2)/prctl(2).
capsicum: rename O_BENEATH_ONLY to O_BENEATH
selftests: rename O_BENEATH_ONLY to O_BENEATH
seccomp: selftest for seccomp_data.tid/tgid

When the build environment allows, check for the
added .tid and .tgid fields.  Include a runtime
check so a test compiled with new system but run
on an old system will fail appropriately.