Dump and restore empty abbrev tables.

Dump and restore empty abbrev tables.

Abbrev tables are obarrays and thus don't print out in a useful form.
They need to be assembled at load time.  Fortunately, loadup.el only
gives us empty abbrev tables, so we don't have to actually restore any
abbrevs, only the tables.

* lisp/loadup.el: When variable values are abbrev tables, emit a
"make-abbrev-table" initialization with the appropriate property
lists.  Check abbrev tables and their parents for instances of
sharing.  Reject any abbrev tables that are not empty.