cp: Add -N flag, inspired by NetBSD's similar flag

Add -N to supress copying of file flags when -p is specified (explicitly
or implicitly). Often times we don't care about the flags or wish to be
able to copy to NFS, and this comes in handy for that. FreeBSD's and
NetBSD's cp are somewhat different, so I had to reimplement all but one
of the patch hunks...

Obtained from: NetBSD (cp.1 1.25, cp.c 1.37, utils.c 1.28 by elad)
Sponsored by: Netflix

Differential Revision: https://reviews.freebsd.org/D42673