elftcl: add -i flag to ignore unknown flags

This may allow an identical elfctl invocation to be used on multiple
FreeBSD versions, with features not implemented on older releases being
silently ignored.

PR: 252629 (related)
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28130