I think it would be useful to spell out the error from
idna_to_ascii_lz(). Strange that libidn doesn't have a suitable
strerror for these codes, so I've added one to Curl's strerror.c

I've not checked if all the Idna_rc codes are present in older
libidn. Only tested with ver 0.4.6.

And I'm puzzled by this block in lib/setup.h:

#ifndef ECONNRESET
#ifdef WSAECONNRESET
#define ECONNRESET WSAECONNRESET
#else
/* This will effectively prevent the code from wo
   aspect, but it still compile fine! */
#define ECONNRESET 10000
#endif
#endif

It's needed nowhere AFAICS, so it couldn't it be deleted?

--gv