stat the pathname "" in order to decide that the pathname "/" is
a directory. This caused `cp kernel /' to fail if the kernel has
the POSIX behaviour of not allowing the pathname "" to be an alias
for ".". It presumably also caused `cp /etc/motd /' to fail in
the unlikely event that "." is not stat'able.
Be more careful about concatenating pathnames: don't check that
the pathname fits until prefixes have been discarded (the check
was too strict). Print the final pathname in error messages.
Terminate the target directory name properly for error messages.
Don't add a slash between components if there is already a slash.
