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