QLocalServer uses user-dependent socket path on macOS

When a short name is passed to QLocalServer::listen function it uses the TMPDIR environment variable (tested on macOS 10.12) to create the full path to the socket file, which is user-dependent and looks like this: /var/folders/xx/hash/T, which depends on the user to which the process belongs.

This differs from Linux (where sockets are created under /tmp directory) and Windows (where they are pipes) in a way that the same short name cannot be used in applications running under different user contexts, for example daemon backend and application frontend.

I think this breaks the general cross-platform design of the QLocalSocket/QLocalServer and requires #ifdef's to use absolute socket paths on macOS.