Fix handling of rights on stdio streams, take two.

Fix handling of rights on stdio streams, take two.

Split the rights-limiting code into two cases: if one of the input
files isn't a regular file, use caph_limit_stream(3) instead of
open-coding the same logic; if both input files are regular files,
and the initial attempts to map them succeed, we limit the rights on
those files to CAP_MMAP_R.

Add a regression test for PR 234885.

PR:		234885
Reviewed by:	delphij
MFC after:	2 weeks
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D19216