sed is rewritten to follow GNU sed 4.9

Bashkit’s sed was rewritten after a comparison with GNU sed 4.9 found 46 mismatches. It adds the missing commands y = l n N P D T z F r R w W, GNU range-address semantics, \U \L \u \l \E case conversion, and real option parsing for -s, -z, -f, -l and attached -i.bak. A missing final newline is preserved, and -i now writes files atomically.

The same release builds VFS paths with forward slashes on every host, including Windows.

See the compatibility scorecard, #2436 and #2435.