U651 - avoid declaring errno unnecessarily

Some changes for standard C platforms, to avoid declaring errno explictly.
Such platforms should declare errno in errno.h, which is already included
in the files in question.

doc/fixes34.3

@@ -34,6 +34,7 @@ Platform- and/or Interface-Specific Fixes
 -----------------------------------------
 win32tty: fix visible CRLF characters during lockfile error message
 win32gui: you couldn't specify an alignment in defaults.nh and have it stick
+unix: don't define errno if NHSTDC
 
 
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