make rsync behave more like GNU cp with regard to file permissions

make rsync behave more like GNU cp with regard to file permissions
when -p (preserve permissions) isn't set.

It works by taking the sending file permissions and masking them with
the umask to create the destination file permissions. (There is really
no "correct" way of doing this but at least we now behave like GNU cp
which fits the principle of least surprise.)

also fixed a race condition in copy_file()