machined: introduce a ptsname_namespace() call and make use of it

The call is like ptsname() but does not assume the pty path was
accessible in the local namespace. It uses the same internal ioctl
though.

src/basic/terminal-util.c

@@ -1074,3 +1074,22 @@ int get_ctty(pid_t pid, dev_t *_devnr, char **r) {
 
         return 0;
 }
+
+int ptsname_namespace(int pty, char **ret) {
+        int no = -1, r;
+
+        /* Like ptsname(), but doesn't assume that the path is
+         * accessible in the local namespace. */
+
+        r = ioctl(pty, TIOCGPTN, &no);
+        if (r < 0)
+                return -errno;
+
+        if (no < 0)
+                return -EIO;
+
+        if (asprintf(ret, "/dev/pts/%i", no) < 0)
+                return -ENOMEM;
+
+        return 0;
+}