pgsql: Introduce PG_MODULE_MAGIC_EXT macro.

Introduce PG_MODULE_MAGIC_EXT macro.

This macro allows dynamically loaded shared libraries (modules) to
provide a wired-in module name and version, and possibly other
compile-time-constant fields in future. This information can be
retrieved with the new pg_get_loaded_modules() function.

This feature is expected to be particularly useful for modules
that do not have any exposed SQL functionality and thus are
not associated with a SQL-level extension object. But even for
modules that do belong to extensions, being able to verify the
actual code version can be useful.

Author: Andrei Lepikhov <lepihov(at)gmail(dot)com>
Reviewed-by: Yurii Rashkovskii <yrashk(at)omnigres(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/dd4d1b59-d0fe-49d5-b28f-1e463b68fa32@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9324c8c580655800331b0582b770e88c01b7a5c4

Modified Files
--------------
contrib/auto_explain/auto_explain.c | 5 +-
contrib/auto_explain/t/001_auto_explain.pl | 13 +++++
doc/src/sgml/func.sgml | 22 +++++++++
doc/src/sgml/xfunc.sgml | 27 ++++++++++
src/backend/commands/extension.c | 53 ++++++++++++++++++++
src/backend/utils/fmgr/dfmgr.c | 67 ++++++++++++++++++++-----
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 7 +++
src/include/fmgr.h | 79 ++++++++++++++++++++++++------
src/tools/pgindent/typedefs.list | 1 +
10 files changed, 248 insertions(+), 28 deletions(-)