Doing a G30 is equivalent to homing the Z-axis

Now sets the axisIsHomed flag for the Z-axis when a G30 command has been
executed successfully, because that puts the Z-axis at a known height.
Web interface improvements

Increase TCP window size from 1 to 2 packets and increase maximum amount
of web data sent to 1460. This decreases the page load time fro a
Windows client fro 6 seconds to less than 1 second. It also fixes a
problem whereby when using a Ubuntu client, file upload and direct print
didn't work unless the reprap.js file was edited to reduce the maximum
amount of data sent per request. Also we now initialize unused memory
with known data so that in response to the M111 S1 command we can better
estimate the amount of free memory.