WL#3304: more DECIMAL digits after the point

Now we support DECIMAL fields with no more than 30 digits after the point.
As internal decimal representation allows up to 65 digits in decimal
it's natural to allow 65 digits after the point as well.

Note added by Trudy Pelzer, 2006-06-02
For a related bug and test case, see BUG#14010, which should be
closed when this work is complete.

The Task
--------
Provide support for DECIMAL/NUMERIC data type values
that allow up to 65 post-decimal digits.

The limitation on 30 digits after the point appears because it's only 5 bits 
reserved in frm file to store this number. This is the 'pack_flags' value of 
frm field data.
It's possible though to use a couple of nearby 'pack_flags' bits for the same 
purpose that is enough to support desirable precision. prepare_create_field 
functions should be modified for that.
Also it's necessary to thoroughly check decimal operations with that numbers
as the value of 31 historically means NOT_FIXED_DEC. Should not be checked
when we operate with DECIMAL's