Extruder: clarify handling of relative/absolute mode on incoming gcode
Extruder: move handling for flowrate outside SOLO mode condition so we can remember previous flowrates
Extruder: append_empty_block returns the new block so we don't have to look it up
Extruder: remove redundant relative/absolute conversions
Extruder: don't clobber target position
Extruder: use block returned from append_empty_block() instead of munging the queue
Extruder: whitespace and comments