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.
Make logmask consistent with scrollback mask
Cosmetics
Added help text for sysinfo plugin.
Merge pull request #585 from rahatarmanahmed/master

Added help text to sysinfo
Merge pull request #582 from Repentinus/patch-1

Support for away tracking regardless of channel size
Fix crash when quickly attaching/detaching tabs
Fixed misleading comments identifying the colours

Replaced incorrect numbers (16 to 31) for mIRC colours with the correct ones (0 to 15).
Merge pull request #588 from Repentinus/patch-2

Fixed misleading comments identifying the colours
Add auto-reconnect to preferences
Add "#define GLIB_DISABLE_DEPRECATION_WARNINGS" to eliminate a warning message