SCP: Assure that debug writes that check errno get a correct value

The original solution provided in response to #957 needs to be adjusted
to reflect that errno is only guaranteed to be set by fwrite() if an error
occurred. otherwise a non zero value may have been set by some other
call elsewhere in the program.  All other cases where errno is explicitly
check in simh, it is only done after receiving a return status from a
system call.

Fix problem reported in #1108

scp.c

@@ -13396,12 +13396,12 @@ static void _debug_fwrite_all (const char *buf, size_t len, FILE *f)
 size_t len_written;
 
 while (len > 0) {
+    errno = 0;
     len_written = fwrite (buf, 1, len, f);
     len -= len_written;
     buf += len_written;
     if (errno == EAGAIN)    /* Non blocking file descriptor buffer full? */
         sim_os_ms_sleep(10);/* wait a bit to retry */
-    errno = 0;
     }
 }