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Use int32_t as type of fd on Mac.

On POSIX the "int" is still 32bit on 64bit machines, so it's safe to use
"int32_t" to repressent "int".

Also note that IntegetValue() returns "in64_t" so it could cause loss of
precision when converting to "int", we should use "Int32Value" instead.
Merge pull request #5 from atom/int-size

Fix possible loss of precision when converting between int and JavaScript integer