Fix handling of rights on stdio streams.

• Limit rights on stdio before opening input files. Otherwise, open() may return one of the standard descriptors and we end up limiting rights such that we cannot read from one of the input files.
• Use caph_limit_stdio(), which suppresses EBADF, to ensure that we don't emit an error if one of the stdio streams is closed.
• Don't bother further limiting rights on stdin when stdin isn't going to be used. Doing so correctly requires checking for a number of edge cases, and it doesn't provide any significant benefit.

PR: 234885
Reviewed by: oshogbo
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18860