Preliminary version for serving all files for Duet

Changed webserver and network code to support multiple concurrent
connections
Removed support for differential-mode ultrasonic z-sensor
Reduced X and Y instantDv from 15 to 10 mm/sec to allow outlines of
holes to e drawn at lower speed
Changed temperature reading code to give thermistor disconnected reading
of -273C when there is a negative high ADC offset correction
Tidied up http/lwip interface

Removed file httpd.c and moved its functions into Network.cpp. This
gives us type-checking across more interface boundaries and eliminates a
number of global functions. Also removed unused files fs.c, fsdata.c,
httpd-original.c.