Allow NickServ to ignore accounts if the user's account create ts is wrong.

Allow NickServ to ignore accounts if the user's account create ts is wrong.

src/nickserv.c (KEY_HANDLE_TS_MODE): Define.
  (enum handle_ts_mode): Define.
  (nickserv_conf): Add handle_ts_mode field.
  (nickserv_conf_read): Set it.
  (handle_account): Use it.
srvx.conf.example: Document it.