Implement archive_timeout feature to force xlog file switches to occur no more

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