MFC r355460: libbe: fix build against sysutils/openzfs, part 1

MFC r355460: libbe: fix build against sysutils/openzfs, part 1

This is the half of the changes required that work as-is with both in-tree
ZFS and the new hotness, sysutils/openzfs.  Highlights are less dependency
on header pollution (from somewhere) and using 'mnttab' instead of
'extmnttab'.   In the in-tree ZFS, the latter is a #define for the former,
but in the port extmnttab is actually a distinct struct that's a super-set
of mnttab.  We really want mnttab here anyways, so just use it.