[PATCH] two-arg current_setting() with fallback

Apologies if this is a double-post.

Enclosed is a patch that creates a two-arg current_setting() function. From the commit message:

The two-arg form of the current_setting() function will allow a
fallback value to be returned instead of throwing an error when an
unknown GUC is provided. This would come in most useful when using
custom GUCs; e.g.:

-- errors out if the 'foo.bar' setting is unset
SELECT current_setting('foo.bar');

-- returns current setting of foo.bar, or 'default' if not set
SELECT current_setting('foo.bar', 'default')

This would save you having to wrap the use of the function in an
exception block just to catch and utilize a default setting value
within a function.