pgsql: Change WAL-logging scheme for multixacts to be more like regular

Log Message:
-----------
Change WAL-logging scheme for multixacts to be more like regular
transaction IDs, rather than like subtrans; in particular, the information
now survives a database restart. Per previous discussion, this is
essential for PITR log shipping and for 2PC.

Modified Files:
--------------
pgsql/doc/src/sgml/ref:
pg_resetxlog.sgml (r1.10 -> r1.11)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/ref/pg_resetxlog.sgml.diff?r1=1.10&r2=1.11)
pgsql/src/backend/access/heap:
heapam.c (r1.193 -> r1.194)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/heapam.c.diff?r1=1.193&r2=1.194)
pgsql/src/backend/access/transam:
multixact.c (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/multixact.c.diff?r1=1.4&r2=1.5)
rmgr.c (r1.18 -> r1.19)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/rmgr.c.diff?r1=1.18&r2=1.19)
xlog.c (r1.197 -> r1.198)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c.diff?r1=1.197&r2=1.198)
pgsql/src/bin/pg_controldata:
pg_controldata.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_controldata/pg_controldata.c.diff?r1=1.24&r2=1.25)
pgsql/src/bin/pg_resetxlog:
pg_resetxlog.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/pg_resetxlog/pg_resetxlog.c.diff?r1=1.33&r2=1.34)
pgsql/src/include/access:
htup.h (r1.74 -> r1.75)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/htup.h.diff?r1=1.74&r2=1.75)
multixact.h (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/multixact.h.diff?r1=1.2&r2=1.3)
xlog.h (r1.64 -> r1.65)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/access/xlog.h.diff?r1=1.64&r2=1.65)
pgsql/src/include:
c.h (r1.184 -> r1.185)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/c.h.diff?r1=1.184&r2=1.185)
pgsql/src/include/catalog:
pg_control.h (r1.22 -> r1.23)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_control.h.diff?r1=1.22&r2=1.23)