Add more BEGIN_DECLS / END_DECLS to <sys/capability.h>.

capability.h currently only wraps some of its declarations in BEGIN_DECLS/END_DECLS, so cap_enter(), cap_sandboxed(), etc. are usable from C++ code but cap_rights_init(), cap_rights_is_valid() etc. are not. This commit fixes this distinction.

Approved by: rwatson (mentor)
MFC after: 1 week
Sponsored by: DARPA, AFRL