RootShell: Register as a Service after starting the shell

This is an experiment to see if it makes the OS less likely to kill off
the AFWall background process.  Ideally we'd like the process to stick
around forever, both to reduce the overhead of Active Rules and so that
we don't miss NFLOG events.
First attempt at fixing NFLOG log output

When using the LOG target, the kernel collects the logs in the background
and dumps them out when we run "dmesg".

But when using the NFLOG target, the "nflog" utility needs to be running
to collect the data in real time.  This isn't currently happening.

This patch starts up a new background shell + service in order to collect
the NFLOG data.  Since the usage of nflog is quite different from dmesg,
Api and LogActivity were modified accordingly.

Note that nflog does NOT currently terminate when AFWall exits.  And only
one instance of nflog can be active at a time.  In a future commit we
should make nflog more sensitive to kills (e.g. terminate when stdin gets
closed).