Allow ircd to run without ircd.conf using sensible defaults

Allow ircd to run without ircd.conf using sensible defaults

- Missing config file prints a warning and continues instead of exiting
- Default listen port is 6667 when no me{} block exists
- Auto-inject allow-all block when no allow{} blocks are configured
- Run from current directory instead of requiring DPATH to exist
- Always allow -f flag for specifying config file path
- Make RPL_POLICY (AUP notice) fully runtime-configurable; skip if empty
- Update convert-conf.py to emit network{}, limits{}, and general{}
  blocks with Serenity-IRC defaults, and fix F-line to use limits{}

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