WIP issue 2094.

Commit 1421c89 expanded the size of a zbookmark_t from 24 to 25 64-bit
values which similarly expands the size of the "scan" entry in the pool's
object directory and causes the pool to become un-importable by other
ZFS implementations.

This patch is a work-in-progress second cut at a fix.  It does two
things: first, replaces the scn_bookmark member of dsl_scan_phys_t
with 4 members representing the members of a zbookmark_t.  Second, it
relaxes the size check during import to alow a broken pool to import by
allowing an EOVERFLOW that occurs when the "scan" entry is too long by
a single integer.