pgsql: Make equalTupleDescs() compare attlen/attbyval/attalign rather

Log Message:
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Make equalTupleDescs() compare attlen/attbyval/attalign rather than
assuming comparison of atttypid is sufficient. In a dropped column
atttypid will be 0, and we'd better check the physical-storage data
to make sure the tupdescs are physically compatible.
I do not believe there is a real risk before 8.0, since before that
we only used this routine to compare successive states of the tupdesc
for a particular relation. But 8.0's typcache.c might be comparing
arbitrary tupdescs so we'd better play it safer.

Modified Files:
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pgsql/src/backend/access/common:
tupdesc.c (r1.110 -> r1.111)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/common/tupdesc.c.diff?r1=1.110&r2=1.111)