pgsql: Add server support for "plugin" libraries that can be used for

Log Message:
-----------
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.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.73 -> r1.74)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.73&r2=1.74)
    pgsql/doc/src/sgml/ref:
        load.sgml (r1.21 -> r1.22)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/load.sgml.diff?r1=1.21&r2=1.22)
    pgsql/src/backend/postmaster:
        postmaster.c (r1.498 -> r1.499)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.498&r2=1.499)
    pgsql/src/backend/tcop:
        postgres.c (r1.498 -> r1.499)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/postgres.c.diff?r1=1.498&r2=1.499)
        utility.c (r1.265 -> r1.266)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/tcop/utility.c.diff?r1=1.265&r2=1.266)
    pgsql/src/backend/utils/fmgr:
        dfmgr.c (r1.87 -> r1.88)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/dfmgr.c.diff?r1=1.87&r2=1.88)
    pgsql/src/backend/utils/init:
        miscinit.c (r1.156 -> r1.157)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c.diff?r1=1.156&r2=1.157)
    pgsql/src/backend/utils/misc:
        guc.c (r1.341 -> r1.342)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.341&r2=1.342)
        postgresql.conf.sample (r1.185 -> r1.186)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.185&r2=1.186)
    pgsql/src/include:
        fmgr.h (r1.45 -> r1.46)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/fmgr.h.diff?r1=1.45&r2=1.46)
        miscadmin.h (r1.187 -> r1.188)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h.diff?r1=1.187&r2=1.188)