[wifi-ap] When sharing is disabled, wifi-ap blindly clears net.ipv4.ip_forward

When the wifi-ap snaps 'share.disabled' key is toggled from 'false' to 'true', wifi-ap will blindly clear the kernel networking parameter 'net.ipv4.ip_forward' via sysctl. This has the potential to break other snaps on the system which may also use IP forwarding.

Perhaps the wifi-ap snap could inspect iptables and determine if additional rules have been added which require forwarding, and not clear ip_forward if so?

This is being reported against wifi-ap snap version 15 (146), and was found by Simon during a discussion of whether or not Docker's iptables rules interfered with wifi-ap's rules (they don't).