finish const-ifying ast_func_read()

channels/chan_zap.c

funcs/func_base64.c

funcs/func_blacklist.c

funcs/func_callerid.c

funcs/func_cdr.c

funcs/func_channel.c

funcs/func_curl.c

funcs/func_cut.c

funcs/func_db.c

funcs/func_enum.c

funcs/func_env.c

funcs/func_global.c

funcs/func_groupcount.c

funcs/func_logic.c

funcs/func_math.c

funcs/func_md5.c

funcs/func_module.c

funcs/func_odbc.c

funcs/func_rand.c

funcs/func_realtime.c