MFC r331329: Fix kernel memory disclosure in ibcs2_getdents

ibcs2_getdents() copies a dirent structure to userland. The ibcs2
dirent structure contains a 2 byte pad element. This element is never
initialized, but copied to userland none-the-less.

Note that ibcs2 has not built on HEAD since r302095.

Submitted by: Domagoj Stolfa <ds815@cam.ac.uk>
Reported by: Ilja Van Sprundel <ivansprundel@ioactive.com>
Security: Kernel memory disclosure (803)
Sponsored by: The FreeBSD Foundation

MFC r282708:
On exec, single-threading must be enforced before arguments space is
allocated from exec_map.

Merge r263233 from HEAD to stable/10:

Update kernel inclusions of capability.h to use capsicum.h instead; some
further refinement is required as some device drivers intended to be
portable over FreeBSD versions rely on __FreeBSD_version to decide whether
to include capability.h.

Sponsored by: Google, Inc.

MFC r282708:
On exec, single-threading must be enforced before arguments space is
allocated from exec_map.

Merge r263233 from HEAD to stable/10:

Update kernel inclusions of capability.h to use capsicum.h instead; some
further refinement is required as some device drivers intended to be
portable over FreeBSD versions rely on __FreeBSD_version to decide whether
to include capability.h.

Sponsored by: Google, Inc.