capsicum: realloc enough space for transferred FDs

capsicum_require_rights() doesn't realloc the capsicum_pending_syscall
contents, so when processing a potential set of file descriptors
being sent over a UNIX domain socket, pre-allocate enough space for
the fd/struct file mappings.
capsicum: use correct POLLHUP constant for process descriptors

POLL_HUP is a SIGPOLL si_code value from siginfo.h; should instead
be using POLLHUP from poll.h.