machined: validate machine names at more places

When enumerating machines from /run, and when accepting machine names
for operations, be more strict and always validate.

Note that these checks are strictly speaking unnecessary, since
enumeration happens only on the trusted /run...

src/basic/util.c

@@ -4913,6 +4913,9 @@ int container_get_leader(const char *machine, pid_t *pid) {
         assert(machine);
         assert(pid);
 
+        if (!machine_name_is_valid(machine))
+                return -EINVAL;
+
         p = strjoina("/run/systemd/machines/", machine);
         r = parse_env_file(p, NEWLINE, "LEADER", &s, "CLASS", &class, NULL);
         if (r == -ENOENT)