pgsql: Make the temporary directory for pgstat files configurable by the

Log Message:
-----------
Make the temporary directory for pgstat files configurable by the GUC
variable stats_temp_directory, instead of requiring the admin to
mount/symlink the pg_stat_tmp directory manually.

For now the config variable is PGC_POSTMASTER. Room for further improvment
that would allow it to be changed on-the-fly.

Modified Files:
--------------
pgsql/src/backend/postmaster:
pgstat.c (r1.178 -> r1.179)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/postmaster/pgstat.c?r1=1.178&r2=1.179)
pgsql/src/backend/utils/misc:
guc.c (r1.465 -> r1.466)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c?r1=1.465&r2=1.466)
postgresql.conf.sample (r1.243 -> r1.244)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/postgresql.conf.sample?r1=1.243&r2=1.244)
pgsql/src/include:
pgstat.h (r1.77 -> r1.78)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/pgstat.h?r1=1.77&r2=1.78)
pgsql/doc/src/sgml:
config.sgml (r1.184 -> r1.185)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/config.sgml?r1=1.184&r2=1.185)
monitoring.sgml (r1.61 -> r1.62)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/monitoring.sgml?r1=1.61&r2=1.62)