Make chflags(1) more chmod(1)-like (and more feature complete):

• Add -v to print file names as they are processed; -vv prints the flags change as well.
• Add -f to ignore failures with the same semantics as chflags(1), neither printing an error nor affecting the return code.
• Don't try to set the flags if they won't change.

I made minor cosmetic tweaks to the code in the patch.

MFC after: 1 week
PR: 112827
Submitted by: Ighighi <ighighi at gmail.com>