serial: introduce local receive data buffer

Add a local RX buffer to the common code of libsigrok's serial layer.
Callers of the serial layer's API won't notice, this is an internal
detail of how alternative transports receive their data from the
physical line, and pass it to read() calls emitted by device drivers.

The libserialport specific code still calls into the library, and does
not use the RX buffer. Future HID and BLE support will use the buffer.