WIP: default values for function parameters

Hello

attached patch allows to define for any PL language default values for
parameters.

Example:
postgres=# create function fx(a int, b int default 30, c int default 40)
postgres-# returns int as $$ select $1 + $2 + $3; $$
postgres-# language sql;
CREATE FUNCTION
postgres=# select fx();
ERROR:  function fx() does not exist
LINE 1: select fx();
               ^
HINT:  No function matches the given name and argument types. You
might need to add explicit type casts.
postgres=# select fx(10);
 fx
----
 80
(1 row)

postgres=# select fx(10,11);
 fx
----
 61
(1 row)

postgres=# select fx(10,11,12);
 fx
----
 33
(1 row)

Know bugs:
blind ambiguous call detection

comments, ideas?

regards
Pavel Stehule