Add erltrek shell command exec function

* Add erltrek_shell:dispatch_and_result/1
  which parse command strings and return the results to stdio

* Update erltrek_shell:server_loop/0 to use the dispatch_and_result/1
Update srscan at starting game to use erltrek_shell:dispatch_and_result/1
Update #shipstate.energy type to non_neg_integer()

* The energy value could be zero
Fix erltrek_galaxy:call/1 return type to term()