Update zsync2 (#1108)
Update zsync2 (#1108)

Resync w/ upstream
Properly handle filesystems that don't support hard-links, like legacy
zsync used to.
Frenzie/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,NiLuJe/koreader-base,Frenzie/koreader-base,koreader/koreader-base,koreader/koreader-base,Frenzie/koreader-base,Frenzie/koreader-base,NiLuJe/koreader-base,koreader/koreader-base,koreader/koreader-base