pgsql: Move pgstat.tmp into a temporary directory under $PGDATA named

Log Message:
-----------
Move pgstat.tmp into a temporary directory under $PGDATA named pg_stat_tmp.
This allows the use of a ramdrive (either through mount or symlink) for
the temporary file that's written every half second, which should
reduce I/O.

On server shutdown/startup, the file is written to the old location in
the global directory, to preserve data across restarts.

Bump catversion since the $PGDATA directory layout changed.

Modified Files:
--------------
pgsql/doc/src/sgml:
monitoring.sgml (r1.60 -> r1.61)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.60&r2=1.61)
storage.sgml (r1.23 -> r1.24)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/storage.sgml?r1=1.23&r2=1.24)
pgsql/src/bin/initdb:
initdb.c (r1.158 -> r1.159)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/bin/initdb/initdb.c?r1=1.158&r2=1.159)
pgsql/src/include/catalog:
catversion.h (r1.474 -> r1.475)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/catversion.h?r1=1.474&r2=1.475)
pgsql/src/backend/postmaster:
pgstat.c (r1.177 -> r1.178)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.177&r2=1.178)