aclitem accessor functions

Dear patchers,

Please find a attached a small patch that adds accessor functions
for "aclitem" so that it is not an opaque datatype.

I needed these functions to browse aclitems from user land. I can load
them when necessary, but it seems to me that these accessors for a backend
type belong to the backend, so I submit them.

I wasn't sure of what oid should be given...
I attributed new numbers at the end of "pg_proc.h".

It validates for me against current cvs head.

Have a nice day,

--
Fabien Coelho - coelho(at)cri(dot)ensmp(dot)fr