cp: Fix dead link case.

The test case was a) not doing what I intended it to do and b) missing
a third possible configuration. Fix the test case, and fix the code
by not setting the beneath flag (which restricts operations to the
destination tree) unless we are recursing.

Fixes: 82fc0d09e862
Sponsored by: Klara, Inc.
Reviewed by: allanjude, markj
Differential Revision: https://reviews.freebsd.org/D50257