errmsg() clobbers errno

Hi All

While debugging an extension I discovered that the errmsg()
function zeros out errno.

This is annoying because if the process of assembling a meaningful
error message happens to call errmsg() before calling strerror()
we lose the strerror information. This is exactly the time when we
want to preserve any available error state.

I am attaching a patch to preserve errno across errmsg() calls.

Does this seem like a good idea?

Best, John