Restructure ngIRCd configuration, introduce [Limits] and [Options]

Restructure ngIRCd configuration, introduce [Limits] and [Options]

The intention of this restructuring is to make the [Global] section much
cleaner, so that it only contains variables that most installations must
adjust to the local requirements.

All the optional variables are moved to [Limits], for configurable limits
and timers of ngIRCd, and [Options], for optional features.

The old variables in the [Global] section are deprecated now, but still
recognized.