revert r340156, restoring sys/sys/capability.h

More time is still needed for ports to accommodate the migration to
capsicum.h.

The header was renamed in 2014 due to concerns about conflicts with with
a draft POSIX.1e capability.h header on other systems and there is (now)
no need for complex autoconf tests for both capability.h and capsicum.h.
Any supported Capsicum-capable system has capsicum.h.

Reported by: antoine
Sponsored by: The FreeBSD Foundation