elfdump: adjust stdout/stderr capabilities

stdio uses fstat and the TIOCGETA ioctl. Also collapse the
cap_rights_limit and new cap_ioctls_limit calls into one if statement.
Errors here are not actionable by the user and distinguishing stdout
from stderr doesn't really have value.

Reported by: kib
Reviewed by: allanjude, bapt
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D7944