pgsql: Restructure LOCKTAG as per discussions of a couple months ago.

Log Message:
-----------
Restructure LOCKTAG as per discussions of a couple months ago.
Essentially, we shoehorn in a lockable-object-type field by taking
a byte away from the lockmethodid, which can surely fit in one byte
instead of two. This allows less artificial definitions of all the
other fields of LOCKTAG; we can get rid of the special pg_xactlock
pseudo-relation, and also support locks on individual tuples and
general database objects (including shared objects). None of those
possibilities are actually exploited just yet, however.

I removed pg_xactlock from pg_class, but did not force initdb for
that change. At this point, relkind 's' (SPECIAL) is unused and
could be removed entirely.

Modified Files:
--------------
pgsql/contrib/userlock:
README.user_locks (r1.2 -> r1.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/userlock/README.user_locks.diff?r1=1.2&r2=1.3)
user_locks.c (r1.14 -> r1.15)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/userlock/user_locks.c.diff?r1=1.14&r2=1.15)
user_locks.h (r1.7 -> r1.8)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/userlock/user_locks.h.diff?r1=1.7&r2=1.8)
pgsql/src/backend/access/heap:
hio.c (r1.54 -> r1.55)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/heap/hio.c.diff?r1=1.54&r2=1.55)
pgsql/src/backend/access/nbtree:
nbtpage.c (r1.82 -> r1.83)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/nbtree/nbtpage.c.diff?r1=1.82&r2=1.83)
pgsql/src/backend/storage/lmgr:
README (r1.15 -> r1.16)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/README.diff?r1=1.15&r2=1.16)
deadlock.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/deadlock.c.diff?r1=1.33&r2=1.34)
lmgr.c (r1.71 -> r1.72)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lmgr.c.diff?r1=1.71&r2=1.72)
lock.c (r1.149 -> r1.150)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/lmgr/lock.c.diff?r1=1.149&r2=1.150)
pgsql/src/backend/utils/adt:
lockfuncs.c (r1.16 -> r1.17)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/lockfuncs.c.diff?r1=1.16&r2=1.17)
pgsql/src/include/catalog:
pg_attribute.h (r1.116 -> r1.117)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_attribute.h.diff?r1=1.116&r2=1.117)
pg_class.h (r1.87 -> r1.88)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/catalog/pg_class.h.diff?r1=1.87&r2=1.88)
pgsql/src/include/storage:
lmgr.h (r1.46 -> r1.47)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lmgr.h.diff?r1=1.46&r2=1.47)
lock.h (r1.84 -> r1.85)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/storage/lock.h.diff?r1=1.84&r2=1.85)