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/libsystemd/sd-device/device-monitor.c

@@ -293,8 +293,9 @@ int device_monitor_enable_receiving(sd_device_monitor *m) {
                 return log_debug_errno(r, "Failed to set address: %m");
 
         /* enable receiving of sender credentials */
-        if (setsockopt(m->sock, SOL_SOCKET, SO_PASSCRED, &const_int_one, sizeof(const_int_one)) < 0)
-                return log_debug_errno(errno, "Failed to set socket option SO_PASSCRED: %m");
+        r = setsockopt_int(m->sock, SOL_SOCKET, SO_PASSCRED, true);
+        if (r < 0)
+                return log_debug_errno(r, "Failed to set socket option SO_PASSCRED: %m");
 
         return 0;
 }

src/libsystemd/sd-netlink/netlink-socket.c

@@ -90,9 +90,9 @@ int socket_bind(sd_netlink *nl) {
         socklen_t addrlen;
         int r;
 
-        r = setsockopt(nl->fd, SOL_NETLINK, NETLINK_PKTINFO, &const_int_one, sizeof(const_int_one));
+        r = setsockopt_int(nl->fd, SOL_NETLINK, NETLINK_PKTINFO, true);
         if (r < 0)
-                return -errno;
+                return r;
 
         addrlen = sizeof(nl->sockaddr);