core: add monitor-connection-files=false and ReloadConnections

core: add monitor-connection-files=false and ReloadConnections

Add a "monitor-connection-files" config option, which can be set to
"false" to disable automatic reloading of connections on file change.

To go with this, add a new ReloadConnections method on
o.fd.NM.Settings that can be used to manually reload connections, and
add an nm-cli command to call it.