replace plugins directory with GUC

About that plugins directory ($libdir/plugins) ... I don't think we ever
really got that to work sensibly. I don't remember the original design
discussion, but I have seen a number of explanations offered over the
years. It's not clear who decides what to put in there (plugin author,
packager, DBA?), how to put it there (move it, copy it, symlink it? --
no support in pgxs), and based on what criteria.

It would seem to be much more in the spirit of things to simply list the
allowed plugins in a GUC variable, like

some_clever_name_here = $libdir/this, $libdir/that

but there is probably a reason why this wasn't done that way in the
first place.

Anyway, the current setup stinks. Can we come up with something better?