New function: area(PATH)...

Attached is a patch that includes the ability to get the area of a
closed path. If the path isn't closed, area() returns NULL.

test=> SELECT p, area(p) from t;
p | area
------------------------------------------+------
((0,0),(1,0),(1,1),(2,1),(2,2),(0,2)) | 3
(1 row)

I haven't teased out the inet_client_addr() patch out of my public
development repo, so that's included as well. No document changes
included because the description of the area() function args is a vague
and all encompassing "object". -sc