Improve prctl decoding

Improve prctl decoding

* configure.ac (AC_CHECK_HEADERS): Add linux/securebits.h.
* prctl.c (sys_prctl): Implement full decoding of all PR_* constants
as defined in linux v3.19.
(sys_arch_prctl): Fix decoding of unknown commands.
* xlat/cap.in: New file.
* xlat/pr_mce_kill.in: New file.
* xlat/pr_mce_kill_policy.in: New file.
* xlat/pr_set_mm.in: New file.
* xlat/pr_tsc.in: New file.
* xlat/pr_unalign_flags.in: New file.
* xlat/secbits.in: New file.
* xlat/prctl_options.in: Remove non-linux constants, add fallback
definitions.
* xlat/seccomp_mode.in: Add fallback definitions.