Rename capability.h to capsicum.h: the original name conflicts with the
draft POSIX.1e capability.h used on some systems (e.g., Linux).  On
FreeBSD, leave a wrapper header so that current code continues to compile.

We will eventually want to deprecate the old header as the presence of a
capability.h may be confusing some configure scripts.

Suggested by:	David Drysdale <drysdale at google.com>
Discussed on:	cl-capsicum-discuss
MFC after:	3 weeks
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.

MFC after:	3 weeks
Update most userspace consumers of capability.h to use capsicum.h instead.

auditdistd is not updated as I will make the change upstream and then do a
vendor import sometime in the next week or two.

MFC after:	3 weeks
Bump __FreeBSD_version to reflect capability.h -> capsicum.h change.

MFC after:	3 weeks