1. Reworked network layer to be more robust in the presence of network
errors (hopefully). In particular, don't release transmit buffers if
they might still be needed to retry sending data because the ACK got
lost.
2. Fixed bug whereby fileinfo command looked for the print height and
filament used if the extensio was .gcode or .gc or .gco but not if it
was .g.
3. When the M25 (pause print) command is received, pause the current
gcode so that commands such as M116 (wait for temperatures) can be
interrupted.
