Three receiver security fixes from the audit:
1. --temp-dir symlink escape (High): file_open_temp_dir() opened the
client-controlled scratch dir with a bare open(), so a symlink planted
under the receive root let a peer redirect receiver scratch files
outside the authorized root. The opened dir is now judged by the REAL
path of its fd (via /proc/self/fd), and any target outside the
authorized receive root is refused with a logged error (EACCES). An
in-root symlink (the EXDEV cross-filesystem fallback case) still works,
and the no-root local batch path is unchanged.
2. setuid/setgid/sticky under SUPER_MODE_OFF (High): the special bits were
applied under --perms (and via --chmod) even when the connection forbade
super-user activities. FileAttrPolicy gains super_permitted, set by
file_attr_policy_from_config() from privilege_super_mode_permitted();
metadata_mode_for_policy(), the symlink path, the special-node creation
path, and the deferred directory-mode apply now strip the special bits
when it is false. Exact rsync semantics are preserved when permitted.
3. daemon umask (Low): daemonize() forced umask(0), so implied parent
directories created without -p were world-writable 0777. Set the
conventional daemon umask 022 instead (rsync never forces 0); -p/-a mode
preservation is unaffected because it restores modes via fchmod.
Tests: new unit tests for file_open_temp_dir confinement and the
masked/unmasked special-bit policy (incl. the --chmod path), a daemon
world-writable-dir regression test, an integration escape test, and a
root-only integration test asserting special bits are masked without
--allow-super. The old cross-filesystem test encoded the vulnerable
behavior (symlink target outside the root) and is replaced by the escape
test; the EXDEV fallback code is retained for in-root links.
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