Implement write-retry in modem_send()

In attempt to address the "Error 11" (EAGAIN) error theat Nelgin sees when
configuring some longer modem init strings on Linux.

This is just a single retry (after a yield) after any modem command char
send failure (for any reason), including the terminating carriage-return.

See issue #662 to details.

src/sexpots/sexpots.c

@@ -613,12 +613,19 @@ BOOL modem_send(COM_HANDLE com_handle, const char* str)
 			if(ch!='^' && ch>='@')	/* ^^ to send an '^' char to the modem */
 				ch-='@';
 		}
-		if(!comWriteByte(com_handle,ch))
-			return FALSE;
+		if(!comWriteByte(com_handle,ch)) {
+			YIELD();
+			if(!comWriteByte(com_handle,ch))
+				return FALSE;
+		}
 	}
 	SLEEP(100);
 	comPurgeInput(com_handle);
-	return comWriteByte(com_handle, '\r');
+	if(!comWriteByte(com_handle, '\r')) {
+		YIELD();
+		return comWriteByte(com_handle, '\r');
+	}
+	return TRUE;
 }
 
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