socket-util: add sockaddr_un_set_path() helper

Properly initializing sun_path from foreign data is not easy, given the
size constraints, and NUL confusion. Let's add a helper function for
this.

src/basic/socket-util.h

@@ -188,3 +188,5 @@ 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);