doc: Update lxc-attach(1)
doc: Update lxc-attach(1)

* Add undocumented options (-v/--set-var, --keep-var, -f/--rcfile)
* Change order of option in SYNOPSIS (-L that is placed after "command")
* Add long options in SYNOPSIS

Signed-off-by: KATOH Yasufumi <512423cb3a4857a38f472862b51e4ab1a1730f06@jazz.email.ne.jp>

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