BREAKING: Add Print::availableForWrite method (#7658)

* Add Print::availableForWrite method

Adds an availableForWrite() method to the Print class, matching current
ArduinoCore-API commit 398e70f188e2b861c10d9ffe5e2bfcb6a4a4f489 .

Hook availableForWrite into the SDFS filesystem (other FSes don't have
this capability built-in).

Fixes #7650

* WiFiClient::availableForWrite proto matching Print

* Fix Netdump signedness warning

* Clean up Serial availableForWrite

This is evidently a breaking change due to the type difference.
Arduino's `availableForWrite` returns an `int`, while the
(multiply-implemented, non-virtual) core `availableForWrite` returned
`size_t`.

cores/esp8266/FS.cpp

@@ -46,6 +46,14 @@ int File::available() {
     return _p->size() - _p->position();
 }
 
+int File::availableForWrite() {
+    if (!_p)
+        return false;
+
+    return _p->availableForWrite();
+}
+
+
 int File::read() {
     if (!_p)
         return -1;

cores/esp8266/FS.h

@@ -57,6 +57,7 @@ public:
     // Print methods:
     size_t write(uint8_t) override;
     size_t write(const uint8_t *buf, size_t size) override;
+    int availableForWrite() override;
 
     // Stream methods:
     int available() override;

cores/esp8266/FSImpl.h

@@ -35,6 +35,7 @@ public:
     virtual bool seek(uint32_t pos, SeekMode mode) = 0;
     virtual size_t position() const = 0;
     virtual size_t size() const = 0;
+    virtual int availableForWrite() { return 0; }
     virtual bool truncate(uint32_t size) = 0;
     virtual void close() = 0;
     virtual const char* name() const = 0;

cores/esp8266/HardwareSerial.h

@@ -150,7 +150,7 @@ public:
     {
         return readBytes((char*)buffer, size);
     }
-    int availableForWrite(void)
+    int availableForWrite(void) override
     {
         return static_cast<int>(uart_tx_free(_uart));
     }

cores/esp8266/Print.h

@@ -75,6 +75,10 @@ class Print {
         inline size_t write(char c) { return write((uint8_t) c); }
         inline size_t write(int8_t c) { return write((uint8_t) c); }
 
+        // default to zero, meaning "a single write may block"
+        // should be overriden by subclasses with buffering
+        virtual int availableForWrite() { return 0; }
+
         size_t printf(const char * format, ...)  __attribute__ ((format (printf, 2, 3)));
         size_t printf_P(PGM_P format, ...) __attribute__((format(printf, 2, 3)));
         size_t print(const __FlashStringHelper *);