Add d_sblockloc to libufs(3) disk structure to allow options to be added.

By making the disk block parameter used by the libufs(3) sbread(3)
function visible, applications using sbread(3) can set their own
addition options such as using the STDSB_NOHASHFAIL request to
say that they want the superblock read to succeed even when
the superblock checkhash is incorrect.

While here also add an error message when a check-hash failure
is detected.