For more than a decade we've used the linuxlist.h for double-linked
lists. Let's also add the hlist (double-linked lists with single
pointer sized head, and the hashtable that builds on top of it.
This reflects the versions included in Linux 5.8 with some modifications
to make them build in userspace (remove RCU versions, adjust for
userspace include files and types, convert to doxygen).
Change-Id: I8ef73a62fe9846ce45058eb21cf999dd3eed5741
