symlink-trust: -k/--copy-dirlinks, -K/--keep-dirlinks, --munge-links (+real -l/--links)

Adds symlink-target transmission (File is_symlink+symlink_target, STATUS_SYMLINK
frame, chunk type 2), munge-links sender containment + receiver-side symmetric
target containment, keep-dirlinks confined dir-symlink following (O_NOFOLLOW
realpath-rechecked), and fixes -l to copy symlinks as symlinks. munge_links +
keep_dirlinks cross the wire; copy_dirlinks client-only. PROTOCOL_VERSION
2.12.0->2.13.0. Review fixes: receiver rejects absolute/.. targets, gated unmunge,
-K O_NOFOLLOW+re-fstat, keep_dirlinks set once at config-accept, rel_buf overflow
fails the walk.

src/shared/file.h

@@ -33,6 +33,27 @@ int file_open_for_read(const char* path);
 bool file_write_to_disk(const char* path, const void* data, unsigned long long data_size,
                         bool inplace, bool sparse);
 
+/* Symlink trust-boundary helpers (Phase 4, symlink wave).  --munge-links
+ * sender-side marker: every transmitted symlink target is prefixed with this
+ * while the flag is on; the receiver strips it to restore the real target. */
+#define SYMLINK_MUNGE_PREFIX "#SYMLINK/"
+
+char* file_symlink_munge(const char* target);
+/* True when a lexical target is relative and contains no ".." component, so it
+ * can never escape the receive root once created beneath it. */
+bool file_symlink_target_contained(const char* target);
+/* Strip a leading SYMLINK_MUNGE_PREFIX from `target` (mutable, in place);
+ * returns true when a marker was removed. */
+bool file_symlink_unmunge(char* target);
+/* Create a symlink at `path` -> `target`, confined below the authorized root
+ * (O_NOFOLLOW parent walk, symlinkat; the target is never followed).  Returns
+ * false when a directory already occupies `path`. */
+bool file_symlink_at_secure(const char* path, const char* target);
+/* --keep-dirlinks (-K) receiver process-wide policy: allow an in-root existing
+ * symlink-to-directory to be followed as a directory. */
+void file_set_keep_dirlinks(bool enable);
+bool file_get_keep_dirlinks(void);
+
 /* A configured fd without a canonical identity deliberately rejects paths. */
 bool file_set_authorized_root(int fd, const char* canonical_path);