show() function

Attached is a patch which (I believe) addresses Peter's concerns with
respect to the previous showguc patch. In this patch, contrib/showguc is
gone, the term GUC is not exposed to user space, and a builtin
show('gucvarname') function returning a single scalar value has been
added, e.g.:

test=# select show('wal_sync_method');
show
-----------
fdatasync
(1 row)

I will work up new table function API examples per Peter's suggestion,
and finish the C API documentation (hopefully before the weekend is
over) in a separate patch.

If there are no further objections, please apply.

Thanks,

Joe