capsicum_helpers: Squash errors from closed fds

Squash EBADF from closed stdin, stdout, or stderr in caph_limit_stdio().
Any program used during special shell scripts may commonly be forked
from a parent process with closed standard stream. Do the common sense
thing for this common use.

Reported by: Iblis Lin <iblis AT hs.ntnu.edu.tw>
Reviewed by: oshogbo@ (earlier version)
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D8657