Use PG_MODULE_MAGIC_EXT in our installable shared libraries.

Use PG_MODULE_MAGIC_EXT in our installable shared libraries.

It seems potentially useful to label our shared libraries with version
information, now that a facility exists for retrieving that.  This
patch labels them with the PG_VERSION string.  There was some
discussion about using semantic versioning conventions, but that
doesn't seem terribly helpful for modules with no SQL-level presence;
and for those that do have SQL objects, we typically expect them
to support multiple revisions of the SQL definitions, so it'd still
not be very helpful.

I did not label any of src/test/modules/.  It seems unnecessary since
we don't install those, and besides there ought to be someplace that
still provides test coverage for the original PG_MODULE_MAGIC macro.

Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/dd4d1b59-d0fe-49d5-b28f-1e463b68fa32@gmail.com