[Chicken-hackers] adds errno->string to posix-common.scm

[Alexej Magura]

The patch attached adds a binding to strerror to posix-common.scm^* (version 4.9.0.1).  The other attachment is the patched file, to make it easier to merge the changes with upstream.  I've already tested the changes: it compiles and works, at least in csi.

^{* strerror is specified by both C89 and C99, so I figure that it's probably available on both *nix and Windows systems, hence adding it in posix-common.scm instead of posixunix.scm or some other file.}

posix-common.scm.new
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posix-common.scm.patch
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