Add option to support away tracking regardless of channel size

As there is option to disable away tracking, it would make sense to treat channel size limit of 0 as infinity for away tracking purposes. These changes attempt to implement this.
Added support for marking nicks that are away in different colour when away_size_max = 0, but tracking enabled.
Merge pull request #582 from Repentinus/patch-1

Support for away tracking regardless of channel size