log: use N/A if getpid() != gettid() when threaded
log: use N/A if getpid() != gettid() when threaded

Sample log output:

    lxc 20161125201943.819 INFO     lxc_start - start.c:lxc_check_inherited:243 - Closed inherited fd: 54.
--> lxc N/A                INFO     lxc_monitor - monitor.c:lxc_monitor_sock_name:178 - using monitor sock name lxc/ad055575fe28ddd5//var/lib/lxc
    lxc 20161125201943.958 DEBUG    lxc_commands - commands.c:lxc_cmd_handler:893 - peer has disconnected
--> lxc N/A                DEBUG    lxc_commands - commands.c:lxc_cmd_get_state:579 - 'lxc-test-concurrent-0' is in 'RUNNING' state
    lxc 20161125201943.960 DEBUG    lxc_commands - commands.c:lxc_cmd_handler:893 - peer has disconnected
    lxc 20161125201944.009 INFO     lxc_start - start.c:lxc_check_inherited:243 - Closed inherited fd: 3.

Signed-off-by: Christian Brauner <48455ab3070520a2d174545c7239d6d0fabd9a83@ubuntu.com>

federicobriata/lxc,GreatFruitOmsk/lxc,hallyn/lxc,GreatFruitOmsk/lxc,hfuCN/lxc,terceiro/lxc,aeris/lxc,thmo/lxc,caglar10ur/lxc,cjwatson/lxc,ss1h2a3tw/lxc,hallyn/lxc,federicobriata/lxc,LynxChaus/lxc,ss1h2a3tw/lxc,cjwatson/lxc,hfuCN/lxc,caglar10ur/lxc,aeris/lxc,ysbnim/lxc,GreatFruitOmsk/lxc,federicobriata/lxc,LynxChaus/lxc,ysbnim/lxc,hallyn/lxc,ss1h2a3tw/lxc,aeris/lxc,terceiro/lxc,caglar10ur/lxc,ysbnim/lxc,cjwatson/lxc,thmo/lxc,terceiro/lxc,LynxChaus/lxc,thmo/lxc,hfuCN/lxc