Add Print::availableForWrite

If available() is in the base Stream class, then availableForWrite() should be in the base Print class

cores/arduino/HardwareSerial.h

@@ -124,7 +124,7 @@ class HardwareSerial : public Stream
     virtual int available(void);
     virtual int peek(void);
     virtual int read(void);
-    int availableForWrite(void);
+    virtual int availableForWrite(void);
     virtual void flush(void);
     virtual size_t write(uint8_t);
     inline size_t write(unsigned long n) { return write((uint8_t)n); }

cores/arduino/Print.h

@@ -22,6 +22,7 @@
 
 #include <inttypes.h>
 #include <stdio.h> // for size_t
+#include <limits.h> // for INT_MAX
 
 #include "WString.h"
 #include "Printable.h"
@@ -58,6 +59,10 @@ class Print
       return write((const uint8_t *)buffer, size);
     }
 
+    // default to zero, meaning "a single write may block"
+    // should be overriden by subclasses with buffering
+    virtual int availableForWrite() { return 0; }
+
     size_t print(const __FlashStringHelper *);
     size_t print(const String &);
     size_t print(const char[]);

cores/arduino/USBAPI.h

@@ -98,7 +98,7 @@ public:
 	virtual int available(void);
 	virtual int peek(void);
 	virtual int read(void);
-	int availableForWrite(void);
+	virtual int availableForWrite(void);
 	virtual void flush(void);
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t*, size_t);