cp: Move the flags around a bit.

• The HLPR flags are grouped together at the beginning because they are the standard flags for programs using FTS. Move the N flag out from among them to its correct place in the sequence.
• The Pflag variable isn't used outside main(), but moving it out lets us skip initialization and keeps it with its friends H, L and R.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D43063