Assign a typmod of -1 to unadorned CHAR and NUMERIC type specs. This

allows casts without specific length requirements to continue to work
as they did before; that is, x::char will not truncate the value of x,
whereas x::char(1) will.  Likewise for NUMERIC precision/scale.
The column length defaults of char(1) and numeric(30,6) are now inserted
in analyze.c's processing of CREATE TABLE.