bug#18592: FFI should have portable access to ‘errno’

[Nala Ginrut]

Hi Mark!
Thanks for all the advices.

Here's the new patch according to your advices.
Include:
1. Added new procedure pointer->procedure-with-errno with
#:return-errno?

Question: Should we make #:return-errno? true in default? This would
make the name *-with-errno more reasonable. At present, it's false in
default.

2. Used scm_cons2

3. Store errno to a local var after ffi_call immediately.

4. Set errno=0 only when #:return-errno? is true.

5. Merged all modifications into one patch.

Comments please.

Best regards.

0001-Added-new-function-pointer-procedure-with-errno-to-r.patch
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