A stale .bak silently disables the re-fetch backup on Windows

[xroche]

back_finalize() moves an existing file aside to <file>.bak before truncating it, so an aborted re-fetch can put the previous copy back. That rename was a bare RENAME, and Windows refuses to rename onto an existing target, so a .bak outliving a killed run made every later re-fetch of that URL fail the rename, take the tmpfile = NULL branch with nothing logged, and truncate the live file with no backup at all. The #77 guard was off for that file, silently and for good. hts_rename_over() (#754) unlinks and retries, which fixes it.

Worth a look, because clobbering a stale .bak is not free. A run killed between the rename-aside and the finalize leaves the previous complete copy in .bak and a partial in the live file, and the next re-fetch now overwrites the good one. It still looks like the right trade: POSIX rename() has behaved exactly this way since #77 landed, so the alternative is leaving Windows with a guard that stays dead until someone deletes the file by hand, and a leftover .bak is engine garbage that nothing advertises or ever restores. The clobber is logged now, so it is at least visible.

Any failure to create the backup is logged too, instead of quietly disabling the safety net. htscache.c's static hts_rename() wrapper and the hand-rolled old.zip unlink at its only call site go the same way, which removes a copy of the unlink-then-rename idiom rather than adding a fifth.

Test 101 plants both kinds of leftover before the update pass: a stale file, which has to be clobbered so the -M abort can still restore the pass-1 copy, and a directory, which cannot be clobbered and has to be reported instead. Only the Windows leg arms the first half, since POSIX clobbers on its own; the second is armed everywhere. Test 37 gains a live update pass so the dead pass rotates onto an existing old.zip, which nothing covered.

Two older bugs on the same path came out of the review and are filed separately: #774 (the .bak name collides with a mirrored file of the same name, reproduced) and #775 (a failed filecreate() on a chunked re-fetch commits the backup away).

Closes #758