pgsql: Make PG_MODULE_MAGIC required in shared libraries that are loaded

[tgl(at)postgresql(dot)org (Tom Lane)]

Log Message:
-----------
Make PG_MODULE_MAGIC required in shared libraries that are loaded into
the server.  Per discussion, there seems no point in a waiting period
before making this required.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        xfunc.sgml (r1.114 -> r1.115)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/xfunc.sgml.diff?r1=1.114&r2=1.115)
    pgsql/src/backend/utils/fmgr:
        dfmgr.c (r1.84 -> r1.85)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/fmgr/dfmgr.c.diff?r1=1.84&r2=1.85)
    pgsql/src/include:
        fmgr.h (r1.44 -> r1.45)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/fmgr.h.diff?r1=1.44&r2=1.45)
    pgsql/src/tutorial:
        funcs.c (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/tutorial/funcs.c.diff?r1=1.14&r2=1.15)