Fix to compile with Visual C++ 2015

[egtra]

Some fixes for Visual C++ 2015.

• Add #undef environ. Because environ is defined (*__p__environ()) in <stdlib.h>.
• Remove snprintf macro. Visual C++ 2015 has snprintf.
• Remove ECONNRESET macro. This is copy from libtelnet-proxy.c to libtelnet-chatd.c.

[thefallentree]

cool

[seanmiddleditch]

I'm against the changing from Unicode to Multibyte. The latter should be considered deprecated.

I'm also unsure about the conditions that were added for the macros. The compiler version is not directly related to the stdlib version, e.g. you can use mingw to compile using Microsoft's headers.

[egtra]

Thank you for response. I agree it.

But condition for #define snprintf _snprintf is needed for Visual C++ 2015. Because Visual C++ 2015 generates following compile error if there is #define snprintf _snprintf.

1>c:\program files (x86)\windows kits\10\include\10.0.10240.0\ucrt\stdio.h(1927): fatal error C1189: #error: Macro definition of snprintf conflicts with Standard Library function declaration

e.g. you can use mingw to compile using Microsoft's headers.

Therefore I use _UCRT instead _MSC_VER for condition.

[seanmiddleditch]

Looks much better. I forgot to ask the first time: could you please rebase this onto the develop branch? Thanks!

[egtra]

Sure. I rebased it. Thank you too!