pgsql: Implement archive_timeout feature to force xlog file switches to

Log Message:
-----------
Implement archive_timeout feature to force xlog file switches to occur no more
than N seconds apart.  This allows a simple, if not very high performance,
means of guaranteeing that a PITR archive is no more than N seconds behind
real time.  Also make pg_current_xlog_location return the WAL Write pointer,
add pg_current_xlog_insert_location to return the Insert pointer, and fix
pg_xlogfile_name_offset to return its results as a two-element record instead
of a smashed-together string, as per recent discussion.

Simon Riggs

Modified Files:
--------------
    pgsql/doc/src/sgml:
        backup.sgml (r2.82 -> r2.83)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.82&r2=2.83)
        config.sgml (r1.74 -> r1.75)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml.diff?r1=1.74&r2=1.75)
        func.sgml (r1.329 -> r1.330)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.329&r2=1.330)
    pgsql/src/backend/access/transam:
        xlog.c (r1.247 -> r1.248)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.247&r2=1.248)
    pgsql/src/backend/postmaster:
        bgwriter.c (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/bgwriter.c.diff?r1=1.26&r2=1.27)
    pgsql/src/backend/utils/misc:
        guc.c (r1.342 -> r1.343)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.342&r2=1.343)
        postgresql.conf.sample (r1.186 -> r1.187)

(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample.diff?r1=1.186&r2=1.187)
    pgsql/src/include/access:
        xlog.h (r1.72 -> r1.73)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.72&r2=1.73)
        xlog_internal.h (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog_internal.h.diff?r1=1.15&r2=1.16)
    pgsql/src/include/catalog:
        catversion.h (r1.349 -> r1.350)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.349&r2=1.350)
        pg_proc.h (r1.420 -> r1.421)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.420&r2=1.421)