testsuite: probe RESOLVE_BENEATH support functionally for the #715 test

testsuite: probe RESOLVE_BENEATH support functionally for the #715 test

Add resolve_beneath_supported() to rsyncfns: it functionally probes whether the
rsync binary can follow an in-tree directory symlink under its secure resolver
(an initial transfer plus a delta update through a dir-symlink, the operation
issue #715 is about). This tracks the actual binary instead of a platform name.

Use it in symlink-dirlink-basis_test.py in place of the SunOS/OpenBSD/NetBSD/
Cygwin name check: it skips on those platforms too, and additionally on
Linux < 5.6, a seccomp-blocked openat2, and the new --disable-openat2 build,
where the portable O_NOFOLLOW fallback rejects the in-tree symlink.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>