added marco ngx_tcp_cycle_get_module_main_conf

If developer writes a tcp module and defines ngx_module_t:init_module field,
this macro will be useful to get main conf of his tcp module from cycle.

    nginx_module_t {
      ngx_int_t (*init_module)(ngx_cycle_t *cycle);
    }

Note that http module has a similar macro (ngx_http_cycle_get_module_main_conf).
Merge pull request #86 from chobits/new_macro

added marco ngx_tcp_cycle_get_module_main_conf