firewall: Allow ICMP

Ref: http://security.stackexchange.com/questions/22711/is-it-a-bad-idea-for-a-firewall-to-block-icmp
Explicitly sets MTU to 1492 for outbound route
nat.sh may run before system resetting some variables, move them to up.sh
munin: Move data root to /tmp, don't waste space of /rw
pdnsd: Rotate debug log
Don't let system upgrade kernel automatically, it will be erased on reboot anyways
Clean apt cache daily