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.

tests/integration/test_daemon.py

@@ -66,6 +66,7 @@ def _pw_hash(password):
 def _write_client_password_file(path, user, password):
     with open(path, "w") as f:
         f.write("%s:%s\n" % (user, password))
+    os.chmod(path, 0o600)
     return path
 
 
@@ -164,6 +165,7 @@ def daemon_env():
         f.write("# daemon credential store (Wave B)\n")
         f.write("alice:%s\n" % _pw_hash(ALICE_PASS))
         f.write("bob:%s\n" % _pw_hash(BOB_PASS))
+    os.chmod(CRED_FILE, 0o600)
 
     # The config's port is a free port chosen per worker; the `daemon` fixture
     # boots on it (the config-port path) and the --dparam override test boots a
@@ -582,6 +584,7 @@ class TestDaemonAuthentication:
         cred_path = os.path.join(TEST_DATA_DIR, "client_empty.pw")
         with open(cred_path, "w") as f:
             f.write("# nothing here\n")
+        os.chmod(cred_path, 0o600)
         try:
             cmd = CLIENT_CMD + ["--source-dir", SOURCE_DIR,
                                 "--dest-dir", "127.0.0.1::files",