uuid is not multi-user aware

[ntuckerxx]

Using /var/tmp/ruby-uuid is unwise on a multi-user system. If /var/tmp is writable but /var/tmp/ruby-uuid is not (due to it being owned by another user who happens to have generated a UUID), UUID.new fails with an EACCESS error on /var/tmp/ruby-uuid.

[davidlee]

it'd be fairly trivial to fix this by appending the userid to the filename if the intended filename is not writable ...

[drbrain]

According to the UUID spec, all users on the same system need to use a single state file to avoid collisions. The default mode for the state file is 0644, but this can be set via UUID::state_file.