OPDS: ignore links with unknown rel values, treat empty rel as rel="subsection".
OPDS: ignore links with unknown rel values, treat empty rel as rel="subsection".

Frenzie/koreader,pazos/koreader,ashhher3/koreader,lgeek/koreader,noname007/koreader,Markismus/koreader,frankyifei/koreader,NickSavage/koreader,mihailim/koreader,mwoz123/koreader,robert00s/koreader,chrox/koreader,Hzj-jie/koreader,apletnev/koreader,Frenzie/koreader,NiLuJe/koreader,koreader/koreader,poire-z/koreader,chihyang/koreader,NiLuJe/koreader,koreader/koreader,poire-z/koreader,ashang/koreader,houqp/koreader