Remove superuser() checks from pgstattuple

Greetings,

Attached is an rebased and updated patch to remove the explicit
superuser() checks from the contrib extension pgstattuple, in favor of
using the GRANT system to control access, and give the admin flexibility
to GRANT access to this function without having to write wrapper
functions, similar to what was been done with the backend functions.

This, naturally, REVOKE's EXECUTE from public on installation for these
functions.

Prior discussion was on this thread:

20160408214511(dot)GQ10850(at)tamriel(dot)snowman(dot)net

but it seemed prudent to open a new thread, to avoid anyone missing that
this isn't about default roles (which was the subject of that thread).

Thanks!

Stephen