nginx http配置文件的解析过程： 1. ngx_conf_read_token读取到http关键字时，调用http/ngx_http.c
static ngx_command_t ngx_http_command[] = {
{ ngx_string("http"),
NGX_MAIN_CONF | NGX_CONF_BLOCK | NGX_CONF_NOARGS,
ngx_http_block,
0,
0,
NULL},
ngx_null_command
};