generate an error if the foreward

declaration of a func does not exactly
match the actual (or another foreward)
declaration. the reason is that when
there are 2 declarations of a function,
the names of the parameters are taken
from the first. thus
   func x(a int);
followed by
   func x(b int) { ... }
will have the parameter named "a" and
"b" will be undefined.

SVN=114788