pgsql: Add support for forcing a switch to a new xlog file; cause such a

Log Message:
-----------
Add support for forcing a switch to a new xlog file; cause such a switch
to happen automatically during pg_stop_backup().  Add some functions for
interrogating the current xlog insertion point and for easily extracting
WAL filenames from the hex WAL locations displayed by pg_stop_backup
and friends.  Simon Riggs with some editorialization by Tom Lane.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        backup.sgml (r2.81 -> r2.82)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/backup.sgml.diff?r1=2.81&r2=2.82)
        func.sgml (r1.328 -> r1.329)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/func.sgml.diff?r1=1.328&r2=1.329)
    pgsql/src/backend/access/transam:
        xlog.c (r1.245 -> r1.246)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.245&r2=1.246)
    pgsql/src/include/access:
        xlog_internal.h (r1.13 -> r1.14)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog_internal.h.diff?r1=1.13&r2=1.14)
    pgsql/src/include/catalog:
        catversion.h (r1.346 -> r1.347)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h.diff?r1=1.346&r2=1.347)
        pg_control.h (r1.29 -> r1.30)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_control.h.diff?r1=1.29&r2=1.30)
        pg_proc.h (r1.419 -> r1.420)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_proc.h.diff?r1=1.419&r2=1.420)