CLEANUP: Modernize C from 90s style to current C

CLEANUP: Modernize C from 90s style to current C

- Convert all K&R function definitions to ANSI C prototypes
- Remove register keyword throughout
- Replace bzero/bcopy/bcmp with memset/memcpy/memcmp
- Replace index/rindex with strchr/strrchr
- Remove extern int errno declarations
- Remove PROTO() macro (K&R/ANSI compatibility shim)
- Convert all fake-variadic functions (p1..p8 pattern) to proper
  stdarg.h variadics: sendto_one, sendto_prefix_one, sendto_serv_butone,
  sendto_channel_butone, sendto_channelops_butone,
  sendto_channelvoice_butone, sendto_match_butone, sendto_all_butone,
  sendto_common_channels, sendto_channel_butserv, sendto_match_servs,
  sendto_ops, sendto_failops, sendto_umode, sendto_failops_whoare_opers,
  sendto_locfailops, sendto_opers, sendto_ops_butone, sendto_opers_butone,
  sendto_ops_butme, sendto_realops, sendto_SNICK_butone,
  sendto_serv_butone_quit, sendto_one_notice, debug, dumpcore

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