shared/validate: Adds IsNetworkMTU function
shared/validate: Adds IsNetworkMTU function

Validates MTU >= 1280 and <= 9202.
Anything below 68 and the kernel doesn't allow IPv4, anything below 1280 and the kernel doesn't allow IPv6.
So to avoid weird issues require an IPv6-compatible MTU as the low value and cap at the max ethernet jumbo frame size.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

stgraber/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,hallyn/lxd,stgraber/lxd,Skarlso/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,stgraber/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,Skarlso/lxd,hallyn/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd