pgsql: Reduce default value of max_prepared_transactions from 50 to 5.

Log Message:
-----------
Reduce default value of max_prepared_transactions from 50 to 5.  This
saves nearly 700kB in the default shared memory segment size, which seems
worthwhile, and it is a feature that many users won't use anyway.  Per
Heikki's argument, there is no point in a compromise value --- those who
are using 2PC at all will probably want it at least equal to max_connections.
But we can't set it to zero by default without breaking the prepared_xacts
regression test.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        runtime.sgml (r1.348 -> r1.349)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.348&r2=1.349)
    pgsql/src/backend/access/transam:
        twophase.c (r1.10 -> r1.11)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c.diff?r1=1.10&r2=1.11)
    pgsql/src/backend/utils/misc:
        guc.c (r1.286 -> r1.287)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.286&r2=1.287)
        postgresql.conf.sample (r1.161 -> r1.162)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.161&r2=1.162)