libpq stricter integer parsing

Follow up on a patch and discussion with Tom, currently integer parsing on 
keywords in libpq is quite loose, resulting in trailing garbage being 
ignored and allowing to hide bugs, eg:

   sh> psql "connect_timeout=2,port=5433"

The timeout is set to 2, and the port directive is silently ignored.
However, URL parsing is stricter, eg on "port".

The attached patch checks integer syntax errors and overflows, and report 
errors.

The pros is that it helps detect bugs. The cons is that some people may 
not want to know about these if it works in the end.

-- 
Fabien.