[base] Counter: some API cleanups
[base] TokenShaper: adapted to changes in Counter API
Merge branch 'master' of github.com:xzero/x0
add support for explicitely specifying reuse_port opt in server config's listen directive.

Example:

    handler setup {
        listen bind: 0.0.0.0, port: 80, reuse_port: true
        worker [0, 1, 2, 3]
        # ...
    }

This will create 4 listener ports (one for each worker) if supported by
the kernel, but will fall back if not.