Multiple additions, bugfixes, minor things

Major change :
    Tracing the request length throughout plugins to allow null bytes in the request (enable gzip compression, images, etc.). This change implies that the C plugin interface get a pointer to the request_size passed in order to be able to know the real request length (without using the misleading strlen()) and to update this length.

Bugfix change :
    Handling multi-connection on ctl_sock by denying it (using accept-close). When trying to connect - at least - twice to the control socket, new connections weren't accept()-ed thus pselect() returning immediately resulting in a silly CPU usage.
    Destroying plugins according to old loaded list, not using the new one which can be different. The plugin-count can changed while reloading the pugin list, thus misleading proxenet_destroy_plugins_vm() which used the newly-created list.

Minor change :
    Using HTTP_TIMEOUT_SOCK instead of hardcoded 5
    Changing help not to show null pointers in compiled modules