Get rid of redundant production in plpgsql grammar.

There may once have been a reason for the intermediate proc_stmts
production in the plpgsql grammar, but it isn't doing anything useful
anymore, so let's collapse it into proc_sect.  Saves some code and
probably a small number of nanoseconds per statement list.

In passing, correctly alphabetize keyword lists to match pl_scanner.c;
note that for "rowtype" vs "row_count", pl_scanner.c must sort on the
basis of the lower-case spelling.

Noted while fooling with a patch to de-reserve more plpgsql keywords.