CLEANUP: Convert HUB from compile-time define to runtime config

CLEANUP: Convert HUB from compile-time define to runtime config

Hub mode is now configured via ircd.conf using the G: (general
config) line:

    G:hub

Default is leaf mode (non-hub) if not set. The setting is
re-evaluated on /rehash, so hub mode can be toggled without
recompiling or restarting the server.

Removes #ifdef HUB / #ifndef HUB from ircd.c, s_serv.c, s_debug.c,
config.h, and setup.h. Adds runtime is_hub global checked in:
- try_connections(): leaf prevents auto-connecting when already linked
- m_server(): leaf rejects incoming server links if already have one
- m_server(): hub sends GLOBOPS on new server link establishment
- m_version(): 'H' serveropts flag shown dynamically based on is_hub

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