interfaces: builtin: Allow writing DHCP lease files to /run/NetworkMa…

…nager/dhcp (#2049)

* Allow writing DHCP lease files to /run/NetworkManager/dhcp

This is needed to allow console-conf to properly detect that an
interface is configured with DHCP in certain cases where the
NetworkManager snap is installed and running on a device before
console-conf is run.

* directories need a trailing slash

* Actually allow writing files inside that directory

* Fix comment to better convey intent

interfaces/builtin/network_manager.go

@@ -73,6 +73,9 @@ network packet,
 # from netplan
 /run/NetworkManager/{,**} r,
 
+# Allow writing dhcp files to a well-known system location
+/run/NetworkManager/dhcp/{,**} w,
+
 # Needed by the ifupdown plugin to check which interfaces can
 # be managed an which not.
 /etc/network/interfaces r,