Allow using CMSG_NXTHDR with -Wcast-align.

If various checks are omitted, the CMSG_NXTHDR macro expands to
  (struct cmsghdr *)((char *)(cmsg) + \
  _ALIGN(((struct cmsghdr *)(cmsg))->cmsg_len))

Although there is no alignment problem (assuming cmsg is properly aligned
and _ALIGN is correct), this violates -Wcast-align on strict-alignment
architectures. Therefore an intermediate cast to void * is appropriate here.

There is no workaround other than not using -Wcast-align.

MFC after:	2 weeks

sys/sys/socket.h

@@ -515,7 +515,7 @@ struct sockcred {
 	  _ALIGN(sizeof(struct cmsghdr)) > \
 	    (char *)(mhdr)->msg_control + (mhdr)->msg_controllen) ? \
 	    (struct cmsghdr *)0 : \
-	    (struct cmsghdr *)((char *)(cmsg) + \
+	    (struct cmsghdr *)(void *)((char *)(cmsg) + \
 	    _ALIGN(((struct cmsghdr *)(cmsg))->cmsg_len)))
 
 /*