Use C99 designated initializers for some structs

Here is a patch to change some struct initializations to use C99-style
designated initializers.  These are just a few particularly egregious
cases that were hard to read and write, and error prone because of many
similar adjacent types.

(The PL/Python changes currently don't compile with Python 3 because of
the situation described in the parallel thread "PL/Python: Remove use of
simple slicing API".)

Thoughts?

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
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