Editorialize a bit on new ProcessUtility() API.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: f8db76e875099e5e49f5cd729a673e84c0b0471b
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2013-04-28T04:18:45Z
Releases: 9.3.0
Editorialize a bit on new ProcessUtility() API.

Choose a saner ordering of parameters (adding a new input param after
the output params seemed a bit random), update the function's header
comment to match reality (cmon folks, is this really that hard?),
get rid of useless and sloppily-defined distinction between
PROCESS_UTILITY_SUBCOMMAND and PROCESS_UTILITY_GENERATED.

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