chmod: match rsync 3.4.1 --chmod and remove mode masking (#293)

- --chmod no longer implies --preserve-perms; repeated --chmod options
  accumulate, and D/F/X selectors plus s/t special bits are supported with
  rsync's exact parse_chmod/tweak_mode semantics.
- Stop masking group/other write and setuid/setgid/sticky: -p copies the
  source mode exactly, no-p new entries use source&~umask, directories keep
  setgid/sticky, and special nodes follow the same rules.
- Apply ownership before mode on the fd path so a chown cannot clear the
  setuid/setgid bits -p just restored (rsync order).
- Update unit and integration tests, including differential checks against
  rsync 3.4.1.

src/client/usage.c

@@ -202,7 +202,8 @@ void print_usage(void) {
   printf("                      --copy-as); --numeric-ids only changes how ids map\n");
   printf("  --no-super          Forbid those super-user activities even when the\n");
   printf("                      receiver is running as root\n");
-  printf("  --chmod <changes>   Modify transferred permissions (rsync syntax)\n");
+  printf(
+      "  --chmod <changes>   Modify new/transferred permissions (rsync syntax; implies no -p)\n");
   printf("  --numeric-ids       Map uid/gid by id instead of by name (a modifier, not\n");
   printf("                      an ownership request: combine with -o/-g or a map)\n");
   printf("  --usermap=MAP       Map usernames when applying ownership: comma-separated\n");