IPTraffic: change NVRAM defaults to 'disabled'

* cstats_enable="0"
IPTraffic/webUI: check/validate minimum size of LAN/subnets only when cstats is enabled

* limit/restrict max size for LAN/subnets on basic-network.asp should only be enforced when IPTraffic/cstats is enabled
* the ipt-account iptables module/match requires LAN subnets to have at least 16 bits set (i.e. class B/equivalent or smaller subnets can be monitored)
* see http://tomatousb.org/forum/t-447639/netmask-must-have-at-least-22-bits-set
* added "Notes" section on admin-iptraffic.asp
IPTraffic: fix backup/restore cstats data via web UI

* allow bigger files to be uploaded/restored via web UI (up to 100k)
* allow more time for cstats to prepare backup do be downloaded via web UI (up to 20 secs)