Invalid recvmmsg detection

The code in netmgr/udp.c uses #ifdef for checking whether the recvmmsg in libuv is available. But because the checked symbols are actually enum members and not preprocessor macros, the checks are always false effectively rendering the recvmmsg support enabled only with libuv >= 1.35.0 where implicit recvmmsg support was added and << 1.37.0 where the recvmmsg use needs to be explicitly enabled with a flag to uv_udp_init_ex().