syscall/receiver: honour a relative alt-basis dir on a daemon receiver (#915)

The symlink-race hardening routed the receiver's basis open through
secure_relative_open(), which rejects any '..' -- so a sibling
--link-dest=../01 on a use-chroot=no daemon was silently ignored and every file
re-transferred (#915/#928, a regression from 3.4.1).

Narrow the confinement to the sanitizing daemon (am_daemon && !am_chrooted) and
re-anchor it at the module root, the real trust boundary: secure_relative_open()
prefixes the cwd's module-relative path (from rsync's logical curr_dir[], a
guaranteed lexical prefix of module_dir) and resolves beneath module_dir, so
RESOLVE_BENEATH permits an in-module '..' climb while still rejecting one that
escapes the module.  secure_basis_open() opens with a bare do_open() in the
non-sanitizing cases.  t_stub.c gains weak curr_dir[]/curr_dir_len for the
helpers (via #pragma weak on non-GNU compilers, where rsync.h erases
__attribute__).

Two tests: link-dest-relative-basis asserts the in-module '..' is honoured;
link-dest-module-escape asserts a --link-dest=../../OUTSIDE climb that leaves
the module is refused (not hard-linked to an outside file).  See upstream
PR #930.

.github/workflows/cygwin-build.yml

@@ -43,8 +43,10 @@ jobs:
       # (rsyncfns.py drives xattrs via getfattr/setfattr from the `attr`
       # package installed above), verified on a real Cygwin host. The real
       # chown/devices tests still skip (need root/mknod), as do the
-      # RESOLVE_BENEATH symlink-race tests.
-      run: bash -c 'RSYNC_EXPECT_SKIPPED=acls-default,acls-depth,acls,bare-do-open-symlink-race,chdir-symlink-race,chown,daemon-access-ip,daemon-chroot-acl,devices,dir-sgid,open-noatime,protected-regular,proxy-response-line-too-long,sender-flist-symlink-leak,simd-checksum,symlink-dirlink-basis make check'
+      # RESOLVE_BENEATH symlink-race tests.  symlink-dirlink-basis also now
+      # RUNS (the #915 non-daemon basis open uses a plain do_open, restoring
+      # following an in-tree dir-symlink basis without RESOLVE_BENEATH).
+      run: bash -c 'RSYNC_EXPECT_SKIPPED=acls-default,acls-depth,acls,bare-do-open-symlink-race,chdir-symlink-race,chown,daemon-access-ip,daemon-chroot-acl,devices,dir-sgid,open-noatime,protected-regular,proxy-response-line-too-long,sender-flist-symlink-leak,simd-checksum make check'
     - name: check (TCP daemon transport)
       # Second run with daemon tests over a real loopback rsyncd; the default
       # 'make check' above uses the secure stdio-pipe transport.