Update static attributes of "content" interface connetions kept in the state when reloading connections.
This is a temporary fix for LP #1825883 and it is limited to "content" interface as this is the only interface where the problem is visible.
The proper fix will be proposed soon and will address all interfaces.
osutil,cmdutil: move CommandFromCore and make it use the snapd snap (if available)
The CommandFromCore helper is currently always looking at the
core snap. With the snapd snap we need to refactor it so that
it uses the snapd snap if available and just falls back to the
core snap.
The osutil.CommandFromCore is not in the rightplace and it also does
not take the snapd snap into account. This PR moves it to the
cmd/cmdutil package (cannot be in cmd because of an import cycle
with cmd/snapinfo.go).
It also tweaks the arguments slightly (no need to pass the
snapMountDir anymore).
