Fix compiler warnings in netty Epoll and unix common

Motivation:
Google requires stricter compilation by adding -Werror and enabling many other warnings.

Modification:

* fix warning caused by -Wmissing-braces

* Use the address of `sendmmsg` rather than the function itself when
checking for presence.  This resovles the warning caused by
`-Wpointer-bool-conversion`.

More detail:
When compiling on Linux, `sendmmsg` is always present, so the
function is always nonnull.  When compiling elsewhere, the
function is defined as `__attribute__((weak))` which means it
may be absent at link time.  This is controlled by
`IO_NETTY_SENDMMSG_NOT_FOUND`, which is off by default.

The reason for the error is due to the risk of accidentally not
calling the function.  By adding `&` before the function, there
is no ambiguity.  (the result of the fn call cannot have its
address taken.)

* use != to check for sendmmsg

Result:
Easier compilation.

transport-native-epoll/src/main/c/netty_epoll_native.c

@@ -315,7 +315,9 @@ static jstring netty_epoll_native_kernelVersion(JNIEnv* env, jclass clazz) {
 }
 
 static jboolean netty_epoll_native_isSupportingSendmmsg(JNIEnv* env, jclass clazz) {
-    if (sendmmsg) {
+    // Use & to avoid warnings with -Wtautological-pointer-compare when sendmmsg is
+    // not weakly defined.
+    if (&sendmmsg != NULL) {
         return JNI_TRUE;
     }
     return JNI_FALSE;

transport-native-unix-common/src/main/c/netty_unix_socket.c

@@ -581,7 +581,7 @@ static jint netty_unix_socket_connectDomainSocket(JNIEnv* env, jclass clazz, jin
 static jint netty_unix_socket_recvFd(JNIEnv* env, jclass clazz, jint fd) {
     int socketFd;
     struct msghdr descriptorMessage = { 0 };
-    struct iovec iov[1] = { 0 };
+    struct iovec iov[1] = { { 0 } };
     char control[CMSG_SPACE(sizeof(int))] = { 0 };
     char iovecData[1];
 
@@ -629,7 +629,7 @@ static jint netty_unix_socket_recvFd(JNIEnv* env, jclass clazz, jint fd) {
 
 static jint netty_unix_socket_sendFd(JNIEnv* env, jclass clazz, jint socketFd, jint fd) {
     struct msghdr descriptorMessage = { 0 };
-    struct iovec iov[1] = { 0 };
+    struct iovec iov[1] = { { 0 } };
     char control[CMSG_SPACE(sizeof(int))] = { 0 };
     char iovecData[1];