Fix TLS connections on MS-Windows

Fix TLS connections on MS-Windows

* src/w32.c (sys_write): If 'send' returns with WSAENOTCONN, and
this is a non-blocking socket whose connection is in progress, set
errno to EWOULDBLOCK, as expected by GnuTLS and other callers.
Avoid overwriting the errno value from 'send' by 'ioctlsocket'.
Suggested by Alain Schneble <a.s@realize.ch>.  (Bug#22789)