derived-mode should provide default abbrev-table

[Andreas Röhler]

Hi,

with resp. to

http://debbugs.gnu.org/cgi/bugreport.cgi?bug=8910

`define-derived-mode' says:

:abbrev-table TABLE
        Use TABLE instead of the default.
        A nil value means to simply use the same abbrev-table as the parent.


Two issues:

- If the slot isn't served, no abbrev-table exists AFAIS. A bug IMHO: 
Specification should be required only for non-defaults. Ie 
":abbrev-table nil" should be the default.

- If a name is given which doesn't represent a valid table, the mode 
hook should make an empty one from it silently.

Cheers,

Andreas