pgsql: Change get_variable_numdistinct's API to flag default estimates

Change get_variable_numdistinct's API to flag default estimates explicitly.

Formerly, callers tested for DEFAULT_NUM_DISTINCT, which had the problem
that a perfectly solid estimate might be mistaken for a content-free
default.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/4c2777d0b733220d9029f78817af8ce671e4a5ad

Modified Files
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src/backend/utils/adt/selfuncs.c | 70 +++++++++++++++++++++++--------------
src/include/utils/selfuncs.h | 3 +-
2 files changed, 45 insertions(+), 28 deletions(-)