security(shared): fix -K TOCTOU, ssh old-args quoting, TLS opts, secret-file perms, sparse dedup

- file: open -K dirlink referents via a race-safe relative O_NOFOLLOW walk
  from the authorized-root fd instead of re-opening an absolute realpath()
  result (removes the intermediate-symlink swap TOCTOU).
- transport_ssh: always single-quote the server path, including --old-args,
  so no mode can inject shell metacharacters.
- transport_tls: set SSL_OP_NO_COMPRESSION and (guarded) SSL_OP_NO_RENEGOTIATION.
- credentials: reject --password-file/--early-input with any group/other
  permission bit; chmod 0600 the affected test fixtures.
- file_store: export file_store_write_sparse() and remove the verbatim
  file.c duplicate.

src/shared/transport_tls.c

@@ -48,6 +48,15 @@ static SSL_CTX* create_ssl_ctx(bool is_server, const char* cert, const char* key
     return NULL;
   }
 
+  /* Harden the context: never negotiate TLS compression (the CRIME attack
+   * vector) and never honour a post-handshake renegotiation request.
+   * SSL_OP_NO_RENEGOTIATION is only available from OpenSSL 1.1.1, so it is
+   * guarded to keep older headers building. */
+  SSL_CTX_set_options(ctx, SSL_OP_NO_COMPRESSION);
+#ifdef SSL_OP_NO_RENEGOTIATION
+  SSL_CTX_set_options(ctx, SSL_OP_NO_RENEGOTIATION);
+#endif
+
   if (SSL_CTX_set_min_proto_version(ctx, TLS1_2_VERSION) != 1) {
     SSL_CTX_free(ctx);
     return NULL;