pgsql: Make the documentation of GUC variables a separate chapter,

Log Message:
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Make the documentation of GUC variables a separate chapter, rather than
a section of the "Server Run-time Environment" chapter. Also, move the
SGML for the new chapter to a separate file and fix the resulting
fallout.

Modified Files:
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    pgsql/doc/src/sgml:
        filelist.sgml (r1.43 -> r1.44)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/filelist.sgml.diff?r1=1.43&r2=1.44)
        postgres.sgml (r1.75 -> r1.76)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/postgres.sgml.diff?r1=1.75&r2=1.76)
        runtime.sgml (r1.352 -> r1.353)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.352&r2=1.353)

Added Files:
-----------
    pgsql/doc/src/sgml:
        config.sgml (r1.13)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?rev=1.13&content-type=text/x-cvsweb-markup)