cp: Improved conformance when copying directories.

• When copying a directory, if the destination exists and is not a directory, we would previously emit an error message and exit. The correct behavior according to POSIX is to emit an error message and continue without descending further into the source directory.

• When copying a directory, if the destination does not exist and we fail to create it, we would previously emit an error message and exit. The correct behavior according to POSIX is to emit an error message and continue. Whether to descend further into the source directory is explicitly left unspecified; GNU cp does not, which seems to me to be the safer and less surprising option, so let's not either.

MFC after: 1 week
Sponsored by: Klara, Inc.
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D44577