define convenient securebits masks for prctl users (v2)

define convenient securebits masks for prctl users (v2)

Hi James, would you mind taking the following into
security-testing?

The securebits are used by passing them to prctl with the
PR_{S,G}ET_SECUREBITS commands.  But the defines must be
shifted to be used in prctl, which begs to be confused and
misused by userspace.  So define some more convenient
values for userspace to specify.  This way userspace does

	prctl(PR_SET_SECUREBITS, SECBIT_NOROOT);

instead of

	prctl(PR_SET_SECUREBITS, 1 << SECURE_NOROOT);

(Thanks to Michael for the idea)

This patch also adds include/linux/securebits to the installed headers.
Then perhaps it can be included by glibc's sys/prctl.h.

Changelog:
	Oct 29: Stephen Rothwell points out that issecure can
		be under __KERNEL__.
	Oct 14: (Suggestions by Michael Kerrisk):
		1. spell out SETUID in SECBIT_NO_SETUID*
		2. SECBIT_X_LOCKED does not imply SECBIT_X
		3. add definitions for keepcaps
        Oct 14: As suggested by Michael Kerrisk, don't
		use SB_* as that convention is already in
		use.  Use SECBIT_ prefix instead.

Signed-off-by: Serge E. Hallyn <serue@us.ibm.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Acked-by: Michael Kerrisk <mtk.manpages@gmail.com>
Cc: Ulrich Drepper <drepper@redhat.com>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: James Morris <jmorris@namei.org>