Check for and use appropriate type of variadic macro arguments.

src/common.h: Define GCC_VARMACROS and C99_VARMACROS where appropriate.
src/chanserv.c (DEFINE_COMMAND):
src/global.c (global_notice): Likewise.
src/helpfile.h (send_channel_message): Likewise.
  (send_channel_notice): Likewise.
  (send_channel_wallops): Likewise.
src/mod-blacklist.c (blacklist_debug): Likewise.
src/mod-helpserv.c (helpserv_msguser): Likewise.
  (helpserv_user_reply): Likewise.
  (helpserv_notice): Likewise.
  (helpserv_notify): Likewise.
  (helpserv_page): Likewise.
  (helpserv_get_page_type): New helper function to reduce size of the
    expanded code for helpserv_page().
src/mod-snoop.c (SNOOP): Expand appropriately for varmacros type.
src/modcmd.h (reply): Likewise.
src/opserv.c (opserv_debug): Likewise.
  (opserv_alert): Likewise.
  (opserv_custom_alert): Likewise.