Adding argument names to aggregate functions

Hi hackers,

I'm sure I'm not the only one who can never remember which way around
the value and delimiter arguments go for string_agg() and has to look it
up in the manual every time. To make it more convenient, here's a patch
that adds proargnames to its pg_proc entries so that it can be seen with
a quick \df in psql.

I also added names to json(b)_object_agg() for good measure, even though
they're more obvious. The remaining built-in multi-argument aggregate
functions are the stats-related ones, where it's all just Y/X (but why
in that order?), so I didn't think it was necessary. If others feel more
strongly, I can add those too.

- ilmari