ltp-networking/nfs: don't use absolute path for showmount.

showmount does not locate at /user/sbin/showmount on Ubuntu 12.04.
ltp-networking/tcp_cmds/finger: fix finger test

finger an unknown host test will return 1 even if execution success
on Ubuntu 12.04
for example:
    $ finger @xxxx || echo "test fail"
    finger: unknown host: xxxx
    getaddrinfo: No address associated with hostname
    test fail
ltp-networking/tcp_cmds/netstat: fix netstat test

netstat cmds will return 1 even if no output in stderr on Ubuntu 12.04
for example:
    $ netstat -s 1>netstat/dev/null 2>&1 ; echo $?
    1
This if condition would have worng result.
    if ! netstat $flag 1>/dev/null 2>&1; then
        end_testcase "netstat $flag failed"
    fi
ltp-networking/tcp_cmds/tcpdump: fix tcpdump test

1. RHOST IP won't appear in network interfaces. It should be LHOST.
2. Host commad can't fetch IP in no DNS enviroment. use ping intead of
   it
ltp-networking/iproute: fix iproute test

ip02: set original mtu size back. this test will set mtu to 300.
ip05: if there is nott only one route using "head" command will get
      wrong result.
ltp-networking/iptables: fix iptables test

"iptables -L -t nat" lists 4 chains not 3 chains on Ubuntu 12.04