pgsql: Fix pageinspect failures on hash indexes.

Fix pageinspect failures on hash indexes.

Make every page in a hash index which isn't all-zeroes have a valid
special space, so that tools like pageinspect don't error out.

Also, make pageinspect cope with all-zeroes pages, because
_hash_alloc_buckets can leave behind large numbers of those until
they're consumed by splits.

Ashutosh Sharma and Robert Haas, reviewed by Amit Kapila.
Original trouble report from Jeff Janes.

Discussion: http://postgr.es/m/CAMkU=1y6NjKmqbJ8wLMhr=F74WzcMALYWcVFhEpm7i=mV=XsOg@mail.gmail.com

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/633e15ea0f1bf2e1d70441fe9da8781befebd6e9

Modified Files
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contrib/pageinspect/hashfuncs.c | 70 +++++++++++++++++++++----------------
src/backend/access/hash/hash_xlog.c | 9 +++++
src/backend/access/hash/hashovfl.c | 17 +++++++--
src/backend/access/hash/hashpage.c | 12 ++++++-
4 files changed, 73 insertions(+), 35 deletions(-)