tree-wide: introduce setsockopt_int() helper and make use of it everywhere

As suggested by @heftig:

https://github.com/systemd/systemd/commit/6d5e65f6454212cd400d0ebda34978a9f20cc26a#commitcomment-30938667

src/basic/socket-util.h

@@ -190,3 +190,10 @@ struct cmsghdr* cmsg_find(struct msghdr *mh, int level, int type, socklen_t leng
 int socket_ioctl_fd(void);
 
 int sockaddr_un_set_path(struct sockaddr_un *ret, const char *path);
+
+static inline int setsockopt_int(int fd, int level, int optname, int value) {
+        if (setsockopt(fd, level, optname, &value, sizeof(value)) < 0)
+                return -errno;
+
+        return 0;
+}