Fix the UDP recvmmsg support

Previously, the netmgr/udp.c tried to detect the recvmmsg detection in libuv with #ifdef UV_UDP_ preprocessor macros. However, because the UV_UDP_ are not preprocessor macros, but enum members, the detection didn't work. Because the detection didn't work, the code didn't have access to the information when we received the final chunk of the recvmmsg and tried to free the uvbuf every time. Fortunately, the isc__nm_free_uvbuf() had a kludge that detected attempt to free in the middle of the receive buffer, so the code worked.

However, libuv 1.37.0 changed the way the recvmmsg was enabled from implicit to explicit, and we checked for yet another enum member presence with preprocessor macro, so in fact libuv recvmmsg support was never enabled with libuv >= 1.37.0.

This commit changes to the preprocessor macros to autoconf checks for declaration, so the detection now works again. On top of that, it's now possible to cleanup the alloc_cb and free_uvbuf functions because now, the information whether we can or cannot free the buffer is available to us.

(cherry picked from commit 73707250)

Closes #3095 (closed)