cp: Fix issues with destination directory mode.

Ensure that we are able to enter the destination directory after we
create it, even if the current umask would normally prevent it, and
that it has the expected permissions once we are done, even if we had
to tweak them to be able to enter it.

Fixes: 82fc0d09e862
Sponsored by: Klara, Inc.
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50266