Return the symbol representing abbrev named ABBREV.
This symbol's name is ABBREV, but it is not the canonical symbol of that name;
it is interned in an abbrev-table rather than the normal obarray.
The value is nil if that abbrev is not defined.
Optional second arg TABLE is abbrev table to look it up in.
The default is to try buffer's mode-specific abbrev table, then global table.
(fn ABBREV &optional TABLE)