cp: Refactor the core logic.

Rewrite copy_file() so the lflag and sflag are handled as early as
possible instead of constantly checking that they're not set and then
handling them at the end. This also opens the door to changing the
failure logic at some future point (for instance, we might decide to
fall back to copying if errno indicates that the file system does not
support links).

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans, allanjude
Differential Revision: https://reviews.freebsd.org/D43055