* 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`.
When building with the Netdump library, the IDE generates the following warning:
> WARNING: library Netdump claims to run on (esp8266 lwip) architecture(s) and may be incompatible with your current board which runs on (esp8266) architecture(s).
Fix the library.properties to call the architecture just "esp8266"
