elfctl: allow features to be specified by value

This will allow elfctl on older releases to set bits that are not yet
known there, so that the binary will have the correct settings applied
if run on a later FreeBSD version.

PR: 252629 (related)
Suggested by: kib
Reviewed by: gbe (manpage, earlier), kib
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28284