correct invalid conditional statement and add usage

Dear all                                                                                                                               
                                                                                                                                       
I correct the invalid conditional parameter and add usages of each function.                                                           
                                                                                                                                       
A short optional parameter of 'h' is set in getopt_long() for help. But '?' is set for conditional parameter at each switch-statement after calling getopt_long().
This correction, '?'->'h', is suited to other programs that requires command-line options in Trema.                                    

And for adding usage, you can see usage of each function, like following.                                                              

```
~/trema$ objects/phost/cli set_host_addr -h                                                                                            
usage: cli set_host_addr [-v] [-h] [--ip_addr ip-address] [--ip_mask netmask] [--mac_addr mac-address]                                 
2012/08/28 19:03:01.764411 [  ERR] (cli.c:139:main) cannot execute.

~/trema$ objects/phost/cli add_arp_entry -h                                                                                            
usage: cli add_arp_entry [-v] [-h] [--ip_addr ip-address] [--mac_addr mac-address]                                                     
2012/08/28 19:03:01.766938 [  ERR] (cli.c:139:main) cannot execute.

~/trema$ objects/phost/cli delete_arp_entry -h                                                                                         
usage: cli delete_arp_entry [-v] [-h] [--ip_addr ip-address]                                                                           
2012/08/28 19:03:01.769496 [  ERR] (cli.c:139:main) cannot execute.                                                                    

~/trema$ objects/phost/cli send_packets -h                                                                                             
usage: cli send_packets [-v] [-h] [--ip_src source ip-address] [--ip_dst destination ip-address]                                       
                                  [--tp_src source transport-address] [--tp_dst destination transport-address]                         
                                  [--n_pkts amount of packet] [--length length of packet]
                                  [--inc_ip_src] [--inc_ip_dst] [--inc_tp_src] [--inc_tp_dst]                                          
                                  [--inc_payload] [--n_pkts amount of packet]
                                  [--duration sending duration]
                                  [--pps packets per second] [--nonblock] [--background]                                               
2012/08/28 19:03:01.772052 [  ERR] (cli.c:139:main) cannot execute.

~/trema$ objects/phost/cli reset_stats -h                                                                                              
usage: cli reset_stats [-v] [-h] [--tx] [--rx]                                                                                         
2012/08/28 19:03:01.774566 [  ERR] (cli.c:139:main) cannot execute.                                                                    

~/trema$ objects/phost/cli show_stats -h                                                                                               
usage: cli show_stats [-v] [-h] [--tx] [--rx]                                                                                          
2012/08/28 19:03:01.777115 [  ERR] (cli.c:139:main) cannot execute.                                                                    
```

Thank you.