Migrate compile-time settings to runtime configuration via ircd.conf

Migrate compile-time settings to runtime configuration via ircd.conf

Move network identity, service names, masking hostnames, tuning limits,
and feature toggles from #defines in config.h to runtime globals in
settings.h/settings.c, parsed from network{}, limits{}, and general{}
blocks in ircd.conf. A single binary can now serve different networks
without recompilation.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>