MINOR: config: Add threads support for "process" option on "bind" lines

MINOR: config: Add threads support for "process" option on "bind" lines

It is now possible on a "bind" line (or a "stats socket" line) to specify the
thread set allowed to process listener's connections. For instance:

    # HTTPS connections will be processed by all threads but the first and HTTP
    # connection will be processed on the first thread.
    bind *:80 process 1/1
    bind *:443 ssl crt mycert.pem process 1/2-