acls/xattrs: -X/--xattrs, -A/--acls, --fake-super

New src/shared/xattr.{c,h}: capture user.* + POSIX ACL xattrs, transmit a bounded
per-file block, re-apply fd-relative. security.*/trusted.*/other system.* never
transmitted/applied (receiver re-validates). Bounds: name<=255 value<=1MiB count
<=256 total<=4MiB. --fake-super records uid:gid:mode:mtime in reserved
user.fastsync.stat (receiver-only). PROTOCOL_VERSION 2.12.0->2.13.0. Review
fixes: reserved key not forwardable, link/hardlink copy-fallback preserves
xattrs, no const-param mutation, per-file warning dedup.

src/client/scanner.h

@@ -19,6 +19,10 @@ typedef struct {
    * capture the source access / birth time into each entry's FileMetadata. */
   bool preserve_atimes;
   bool preserve_crtimes;
+  /* Phase 4 xattrs: when preserve_xattrs || preserve_acls is set the scanner
+   * captures each regular file's whitelisted xattr set onto the File. */
+  bool preserve_xattrs;
+  bool preserve_acls;
   unsigned long long chunk_size;
   char** exclude_patterns;
   int exclude_count;
@@ -87,6 +91,8 @@ typedef struct {
   bool use_metadata;
   bool preserve_atimes;
   bool preserve_crtimes;
+  bool preserve_xattrs;
+  bool preserve_acls;
   unsigned long long chunk_size;
   char** exclude_patterns;
   int exclude_count;