pgsql: Add a feature for automatic initialization and finalization of

Log Message:
-----------
Add a feature for automatic initialization and finalization of dynamically
loaded libraries: call functions _PG_init() and _PG_fini() if the library
defines such symbols.  Hence we no longer need to specify an initialization
function in preload_libraries: we can assume that the library used the
_PG_init() convention, instead.  This removes one source of pilot error
in use of preloaded libraries.  Original patch by Ralf Engelschall,
preload_libraries changes by me.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.72&r2=1.73)
        xfunc.sgml (r1.115 -> r1.116)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.115&r2=1.116)
    pgsql/src/backend/postmaster:
        postmaster.c (r1.497 -> r1.498)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/postmaster.c.diff?r1=1.497&r2=1.498)
    pgsql/src/backend/utils/fmgr:
        dfmgr.c (r1.86 -> r1.87)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/dfmgr.c.diff?r1=1.86&r2=1.87)
    pgsql/src/backend/utils/init:
        miscinit.c (r1.155 -> r1.156)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/init/miscinit.c.diff?r1=1.155&r2=1.156)
    pgsql/src/include:
        miscadmin.h (r1.186 -> r1.187)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/miscadmin.h.diff?r1=1.186&r2=1.187)
    pgsql/src/include/postmaster:
        postmaster.h (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/postmaster/postmaster.h.diff?r1=1.13&r2=1.14)
    pgsql/src/pl/plperl:
        plperl.c (r1.112 -> r1.113)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plperl/plperl.c.diff?r1=1.112&r2=1.113)
    pgsql/src/pl/plpgsql/src:
        pl_handler.c (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_handler.c.diff?r1=1.29&r2=1.30)
        plpgsql.h (r1.77 -> r1.78)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.77&r2=1.78)
    pgsql/src/pl/plpython:
        plpython.c (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpython/plpython.c.diff?r1=1.84&r2=1.85)
    pgsql/src/pl/tcl:
        pltcl.c (r1.105 -> r1.106)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/tcl/pltcl.c.diff?r1=1.105&r2=1.106)