Currently passing data around snap-confine is not very easy because of
historically organic growth of the codebase. To simplify some of that
I'd like to introduce sc_invocation. An ad-hoc structure that captures
some of the variables that participate in the essential part of
snap-confine's logic.

The purpose of this patch is to introduce this structure, switch some of
the code to use it (as opposed to accessing the local variables) and
set the stage for a subsequent patch that moves a large chunk of code
into the two new stubs: enter_{,non_}classic_execution_environment().

This should eventually allow us to make a single call to
sc_should_use_normal_mode() and simply pass the result around via the
invocation structure. There is plenty of more refactoring that can be
done after this patch is in place.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

Handling uid/gid is a bit more delicate as those values routinely change
thought execution. Instead of passing them around in the invocation
structure pass them explicitly to the only place that needs it for now.

Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@canonical.com>

After a good night's sleep I realised I should not have put it there
in the first place. This will allow the invocatoin to collect and contain
only the parts of the arguments that are related to the invocation of
snap-confine and leave out certain things that have more complex state
(uids) or are an implementation detail (apparmor struct) out of it.

Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>

…nvironment

The classic path is (for now) empty so let's not use the invocation
there just yet.


Signed-off-by: Zygmunt Krynicki <me@zygoon.pl>