cp: Address style issues from external review.

• Correctly indent some comment blocks.
• Avoid using strcpy(), even when we know it's safe.
• Drop the (void) casts from fts_set() calls which can never fail.

Fixes: 82fc0d09e862
Sponsored by: Klara, Inc.
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D50256