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

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.