[ADC-Ext 1.0.6] INF field; FS for Free Slots

[Pretorian]

This is already implemented in ApexDC++, but lacks a proper spec;

=== FS - Free slots
This is an extension that will broadcast the amount of free slots the client has available.

Field FS in the client's INF:
[options="autowidth"]
|=====
|FS |Free slots available.
|=====

(No, there should not be a "don't send this too often" notice. It's just like any INF field.)

[Pretorian]

Pushed in official ADC-Ext 1.0.6.