The below range loop will not stop when encountering
the first '.' character in a Darwin version string like "15.6.0".

for i = range osver {
   if osver[i] != '.' {
         continue
      }
   }
}

Therefore, the condition i > 2 was always satisfied and
supportsCloseOnExec was always set to true.

Since the minimum supported version of OSX for go is currently 10.8
and O_CLOEXEC is implemented from OSX 10.7 on the detection code
can be removed and support for O_CLOEXEC is always assumed to exist.

Change-Id: Idd10094d8385dd4adebc8d7a6d9e9a8f29455867
Reviewed-on: https://go-review.googlesource.com/37193Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>