Add server support for "plugin" libraries that can be used for add-on tasks

such as debugging and performance measurement.  This consists of two features:
a table of "rendezvous variables" that allows separately-loaded shared
libraries to communicate, and a new GUC setting "local_preload_libraries"
that allows libraries to be loaded into specific sessions without explicit
cooperation from the client application.  To make local_preload_libraries
as flexible as possible, we do not restrict its use to superusers; instead,
it is restricted to load only libraries stored in $libdir/plugins/.  The
existing LOAD command has also been modified to allow non-superusers to
LOAD libraries stored in this directory.

This patch also renames the existing GUC variable preload_libraries to
shared_preload_libraries (after a suggestion by Simon Riggs) and does some
code refactoring in dfmgr.c to improve clarity.

Korry Douglas, with a little help from Tom Lane.

src/backend/tcop/utility.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/tcop/utility.c,v 1.265 2006/08/12 20:05:56 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/tcop/utility.c,v 1.266 2006/08/15 18:26:58 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -886,12 +886,9 @@ ProcessUtility(Node *parsetree,
 			{
 				LoadStmt   *stmt = (LoadStmt *) parsetree;
 
-				if (!superuser())
-					ereport(ERROR,
-							(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-							 errmsg("must be superuser to do LOAD")));
 				closeAllVfds(); /* probably not necessary... */
-				load_file(stmt->filename);
+				/* Allowed names are restricted if you're not superuser */
+				load_file(stmt->filename, !superuser());
 			}
 			break;