slave: chdir to / after spawning the client application

With this patch, the terminal process now changes PWD to / after
spawning the client application.

This ensures the terminal process itself does not "lock" a
directory. For example, we may keep a mount point from being
unmounted.

Closes #1528

slave.c

@@ -401,6 +401,14 @@ slave_spawn(int ptmx, int argc, const char *cwd, char *const *argv,
         break;
 
     default: {
+
+        /*
+         * Don't stay in CWD, since it may be an ephemeral path. For
+         * example, it may be a mount point of, say, a thumb drive. Us
+         * keeping it open will prevent the user from unmounting it.
+         */
+        chdir("/");
+
         close(fork_pipe[1]); /* Close write end */
         LOG_DBG("slave has PID %d", pid);