Avoid logging pre-verstring EPIPE from sharing downstreams.

If you use the new 'plink -shareexists' feature, then on Unix at least
it's possible for the upstream to receive EPIPE, because the
downstream makes a test connection and immediately closes it, so that
upstream fails to write its version string.

This looks a bit ugly in the upstream's Event Log, so I'm making a
special case: an error of 'broken pipe' type, which occurs on a socket
from a connection sharing downstream, before we've received a version
string from that downstream, is treated as an unusual kind of normal
connection termination and not logged as an error.

windows/winstuff.h

@@ -79,6 +79,8 @@ struct FontSpec *fontspec_new(const char *name,
 #define PLATFORM_HAS_SMEMCLR /* inhibit cross-platform one in misc.c */
 #endif
 
+#define BROKEN_PIPE_ERROR_CODE ERROR_BROKEN_PIPE   /* used in sshshare.c */
+
 /*
  * Dynamically linked functions. These come in two flavours:
  *