bhyveload: add CAP_SEEK to our dirfd rights
In the case of hostbase_fd, this is infact a bug fix; we have a seek
callback that the host: filesystem may use in loader, and we really
don't have a good excuse to break it.
bootfd-derived fds will only be used with fdlopen(3) and rtld doesn't
seem to need pread / lseek at all for it today, but there's no reason to
break if it finds a good reason to later.
Suggested by: markj
(cherry picked from commit cef5e56f3fea33d6c421276af49f2967453ad4ff)