SCP: Slow down debug output writes when the debug fd is non blocking

This problem only appears when debug output is prodigious.  That
prodigious activity is already dramatically affecting timing, so adding
additional delays to allow the debug output to catch up won't
make anything worse.

It appears that we could jump over the C runtime implementation of
fwrite() and do an explicit write() system call and retry that until it
succeeds, but this approach would have two negative consequences:
1) it would jump over other buffered data that the C runtime fwrite()
    may have pending due to output produced by other than debug
    activities, thus emitting output out of order.
2) Windows doesn't have a direct system call used by its C runtime
    for write(), but merely implements write() as part of the C runtime
    and as it turns out that write() returns an int vs a ssize_t type
    result.  An explicit cast could address this, but point 1 would still
    be a concern.

As discussed in #957

scp.c

@@ -13271,6 +13271,8 @@ while (len > 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;
     }
 }