pgsql: Make sure float4in/float8in accept all standard spellings of "in

Make sure float4in/float8in accept all standard spellings of "infinity".

The C99 and POSIX standards require strtod() to accept all these spellings
(case-insensitively): "inf", "+inf", "-inf", "infinity", "+infinity",
"-infinity". However, pre-C99 systems might accept only some or none of
these, and apparently Windows still doesn't accept "inf". To avoid
surprising cross-platform behavioral differences, manually check for each
of these spellings if strtod() fails. We were previously handling just
"infinity" and "-infinity" that way, but since C99 is most of the world
now, it seems likely that applications are expecting all these spellings
to work.

Per bug #8355 from Basil Peace. It turns out this fix won't actually
resolve his problem, because Python isn't being this careful; but that
doesn't mean we shouldn't be.

Branch
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REL9_0_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/32b5a969e9f83866b2b1538e92bd6f1447ec6ac6

Modified Files
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src/backend/utils/adt/float.c | 106 +++++++++++++++++++++++++++++++----------
1 file changed, 81 insertions(+), 25 deletions(-)