pgsql: Add a trace_sort option to help with measuring resource usage of

Log Message:
-----------
Add a trace_sort option to help with measuring resource usage of external
sort operations.  Per recent discussion.  Simon Riggs and Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        config.sgml (r1.23 -> r1.24)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.23&r2=1.24)
    pgsql/src/backend/utils/misc:
        guc.c (r1.289 -> r1.290)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.289&r2=1.290)
    pgsql/src/backend/utils/sort:
        tuplesort.c (r1.50 -> r1.51)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/sort/tuplesort.c.diff?r1=1.50&r2=1.51)
    pgsql/src/include:
        pg_config_manual.h (r1.16 -> r1.17)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/pg_config_manual.h.diff?r1=1.16&r2=1.17)