Use designated initializers for iovec and msghdr structs

POSIX does not guarantee the absence of other members (or even the
ordering of the members) for these structs.

luastatus reportedly does not build with musl because musl inserts
"padding" members into struct msghdr:
https://git.musl-libc.org/cgit/musl/commit/?id=7168790763cdeb794df52be6e3b3

Reported-by: #73

Author: shdown

plugins/network-linux/network.c

@@ -332,8 +332,8 @@ static bool interact(LuastatusPluginData *pd, LuastatusPluginRunFuncs funcs)
     while (1) {
         make_call(pd, funcs, false);
 
-        struct iovec iov = {buf, NBUF};
-        struct msghdr msg = {NULL, 0, &iov, 1, NULL, 0, 0};
+        struct iovec iov = {.iov_base = buf, .iov_len = NBUF};
+        struct msghdr msg = {.msg_iov = &iov, .msg_iovlen = 1};
         ssize_t len = recvmsg(fd, &msg, 0);
         if (len < 0) {
             if (errno == EINTR) {