elfctl: allow features to be specified by value

[emaste]

Summary

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.

Suggested by: kib
Sponsored by: The FreeBSD Foundation

[gbe]

LGTM from manpages

[kib]

usr.bin/elfctl/elfctl.c

264

Do you want to check that only single bit is set in the val?

[emaste]

usr.bin/elfctl/elfctl.c

264

I didn't do that semi-intentionally, so that you could use e.g. elfctl -e =0x81.

[emaste]

Add example of combining numerical values to man page