Make --symlink idempotent

When creating symlinks specified with --symlink, check errno if
creation fails. If the reason was EEXIST, check the path of the
existing symlink. If it matches what was desired, continue as
normal.

bubblewrap.c

@@ -1474,7 +1474,20 @@ setup_newroot (bool unshare_pid,
         case SETUP_MAKE_SYMLINK:
           assert (op->source != NULL);  /* guaranteed by the constructor */
           if (symlink (op->source, dest) != 0)
+          {
+            if (errno == EEXIST)
+            {
+              char existing[PATH_MAX] = "";
+              readlink (dest, existing, sizeof (existing));
+              if (strcmp(existing, op->source) == 0)
+              {
+                break;
+              }
+              die_with_error ("Can't make symlink at %s (existing symlink points to %s)",
+                              op->dest, existing);
+            }
             die_with_error ("Can't make symlink at %s", op->dest);
+          }
           break;
 
         case SETUP_SET_HOSTNAME: