execute_cmd doesn't use return value for ending, just use exit()
modify some constant string sizing and use snprintf for protection
modify ras to concurrent, multi-process server.

- mother wait for child when recieve SIG_CHID.
- execute_cmd use return value instead of exit() for ending now.
add lose signal.h header
change command parsing function to parsed_cmd's member function
one line cmd add cmd number to 4096, modify litte code-style