chdir() to correct path after calling chroot()

Since chroot() doesn't chdir() by default, we are left in
an unreachable directory in node.pexec() (and in xterms.)

fixes #370

mnexec.c

@@ -99,6 +99,7 @@ int main(int argc, char *argv[])
     char path[PATH_MAX];
     int nsid;
     int pid;
+
     static struct sched_param sp;
     while ((c = getopt(argc, argv, "+cdnpa:g:r:vh")) != -1)
         switch(c) {
@@ -156,12 +157,18 @@ int main(int argc, char *argv[])
             sprintf(path, "/proc/%d/ns/mnt", pid);
             nsid = open(path, O_RDONLY);
             if (nsid < 0 || setns(nsid, 0) != 0) {
-                /* Plan B: chroot into pid's root file system */
+                char *cwd = get_current_dir_name();
+                /* Plan B: chroot/chdir into pid's root file system */
                 sprintf(path, "/proc/%d/root", pid);
                 if (chroot(path) < 0) {
                     perror(path);
                     return 1;
                 }
+                /* need to chdir to correct working directory */
+                if (chdir(cwd) != 0) {
+                    perror(cwd);
+                    return 1;
+                }
             }
             break;
         case 'g':