Move refhashes to usr/src/common
This is an upstream of three related Joyent changes:
OS-3948 refhash could be used outside of mpt_sas
OS-7870 Bring refhashes to user land
OS-7358 Would like a refhash walker
The end result is refhashes are available to the kernel via
#include <sys/refhash.h> or via
librefhash (though we will keep the library private to illumos-gate initially) along with an mdb walker.